Yes. You are exactly correct and that is what makes this something that would be VERY unusual for the GMAT. On a normal cause and effect question you would be looking for instances where the cause did not lead to the effect, this would weaken. For example - If I say "Apples are the cause that makes me ill" you could weaken this by showing me instances were apples did not make me ill. That would put a doubt out there of whether this really is the cause of my getting ill. This is what C does! And this would normally be correct (which is why most LSAT takers chose this...

However, with the presence of "only" in this question it is more like a "this is the only possible cause question." So we cannot weaken as we normally would. We cannot show that the cause does not lead to the conclusion because it DOES NOT HAVE TO! If I say the only way for you to get a perfect GMAT score is to study hard - you can see that an example of someone that studied hard and did not get a perfect score does not weaken this. To weaken you have to show me a perfect score from someone who did not study hard. In other words show me that there is another way to achieve the perfect score (like some form of cheating). This is what D does in this problem. It shows that there is another way to gain market share and that purchasing competitors is not the only way.

David@VeritasPrep : that's wonderful. At least, i am assured that I have not been studying in wrong direction. As you say , conclusion is the king. And one should pay careful attention to words in conclusion especially qualifiers. In fact, after analyzing OG questions, I can't stress enough the importance of this statement.

There is a question in PowerScore that had always troubled me earlier , but looks today I have got the answer.

Medical Researcher: As expected, records covering the last four years of ten major hospitals indicate that babies born prematurely were more likely to have low birth weights and to suffer from health problems than were babies not born prematurely. These records also indicate that mothers who had received adequate prenatal care were less likely to have low birth weight babies than were mothers who had received inadequate prenatal care. Adequate prenatal care, therefore significantly decreases the risk of low birth weight babies.

Which one of the following, if true most weakens the medical researcher's argument?

a. The hospital records indicate that many babies that are born with normal birth weights are born to mothers who had inadequate prenatal care.
b. Mothers giving birth prematurely are routinely classified by hospitals as having received inadequate prenatal care when the record of the care is not available.
c. The hospital record indicates that low birth weight babies were routinely classified as having been born prematurely.
d. Some babies not born prematurely whose mothers received adequate prenatal care , have low birth weights.
e. Women who receive adequate prenatal care are less likely to to give birth prematurely than are women who do not receive adequate prenatal care

I could get to the right answer and know this common flaw as well. However, I was never fully convinced why A is wrong except that B is better.

Now I see why A is wrong. Because "significantly decreases" is there in conclusion,leaving possibility for some cases where adequate parental care will not lead to low birth rate babies.

I can't thank you enough David. merci beacoup !

You are very welcome! It makes it worthwhile for me when I hear things like that...let's keep at it!

@David: While trying to master a bit conditional sentences -- ones you explained above-- , i came across the question below from LSAT:

An LSAT question:
It is a principle of economics that a nation can experience economic growth ONLY when consumer
confidence is balanced with a small amount of consumer skepticism.

Correct answer: Any nation in which the prevailing attitude of consumers is exclusively confidence will not experience economic growth.

i am comfortable in its interpreation as you have explained above. Necessary thing is given in the above sentence that is a condition -- ONLY when consumer confidence is balanced with a small amount of consumer skepticism.

My question: Let's remove ONLY from the question stem above:

It is a principle of economics that a nation can experience economic growth when consumer
confidence is balanced with a small amount of consumer skepticism.

So i can write this as follows as in GMAT cause and efect question:
Consumer confidence (CC) + consumer skepticism (CS)=> nation can experience economic growth.

CAN already leaves possibility of not experiencing economic growth i undrstand . It is not like 'will' or 'must' .

Could you suggest now what we can infer from this question if it were a GMAT question (infer or causal reasoning)?

1. even if both CS and CC are there balanced, it is possible that nation does not experience economic growth.
2. If same answer as given above for the LSAT question , would that anser still be true ?

What would you advise to deal with such cases that is conditional and causal. Sine I have known both, I am mixing up a little. I am looking for a boundary line that would help distinguish one from the other and act accordingly.

Thanks for your help on this.

GMATMadeEasy -

It is funny that you picked this particular LSAT problem - not only is it one of my favorite to teach, but it is VERY LSAT!

The statement that you have quoted is designed to list the conditions required for economic growth to be POSSIBLE.

That is why the word "can" is used. The word "can" as you have noted is very ambiguous. How do we take this? Does it mean that it might happen or might not? Actually this is what it means but this does not allow you to draw very many conclusions.

The way the original is worded is that something CAN HAPPEN ONLY IF...this is the way in which "can" is in the strongest position. Because of course if the conditions do not happen then the - in this case economic growth - cannot take place.

If you take out the "only if" the statement does not allow for much a conclusion to be drawn at all. You are left with "It is a principle of economics that a nation can experience economic growth when consumer confidence is balanced with a small amount of consumer skepticism." This is like saying that "my team can win the game if we score 4 goals." But of course we could still lose the game if the other team scores 5. Similarly there are other factors at work in economic growth beyond consumer confidence. So we really cannot draw the conclusion here.

Moral of this story is that "can only" is a strong statement because it gives you a required condition. Whereas "can" is not as strong of a statement.

As you have said below we could have growth or maybe not depending.

I would say that you are getting DEEP into formal logic here and that this is an example of an LSAT question that is done with a specific 5 -step technique that (and this is the cool part) does not even require you to read the answer choices fully - just the last few words!!!

There is nothing like this form of principle question on the GMAT.

Thanks David. I will stop here on conditional sentences then as it is not much relevant to GMAT further down. I thought it might help in RC for MUST BE true but i guess it is not worth its time.

Just last question - the above statement after removing ONLY is not a causal statement but a general statement. I guess, i am/was pressing too hard to make it causal .