Perhaps an MGMAT expert with access to official explanations can comment, but I'd have never chosen A. A directly shows that the author's counter evidence is out of scope, thereby weakening the argument.
In my mind, C is the only possible answer. Let's discuss!
If you immediately classified C as a weakener because it points out that the sample group is narrow, that wasn't a bad first instinct and is usually great thinking - it just doesn't happen to work on this particular question.
Why are narrow sample sizes often weakeners? Because a narrow sample size often indicates unrepresentativeness.
However, let's look at the actual sample: rich old guys.
This particular sample size, if anything, strengthens the author's argument. We're not looking at random poor folk who don't have a choice about where to get treatment (i.e. they have to take the free stuff they can get), we're looking at people who actually have a choice about where to get help. Since the sample is a group that actually has a choice, that choice is useful in determining whether the US has good medical care.
So, there are two ways to think of C:
1) people who have a choice pick US medical care: strengthens the argument; or
2) people who have a choice pick US medical care, but what does that have to do with a vaccine lab's relationship to protection from infection diseases?: outside the scope.
Regardless of whether it strengthens or is outside the scope, it certainly doesn't weaken: pick C.
In my mind, C is the only possible answer. Let's discuss!
If you immediately classified C as a weakener because it points out that the sample group is narrow, that wasn't a bad first instinct and is usually great thinking - it just doesn't happen to work on this particular question.
Why are narrow sample sizes often weakeners? Because a narrow sample size often indicates unrepresentativeness.
However, let's look at the actual sample: rich old guys.
This particular sample size, if anything, strengthens the author's argument. We're not looking at random poor folk who don't have a choice about where to get treatment (i.e. they have to take the free stuff they can get), we're looking at people who actually have a choice about where to get help. Since the sample is a group that actually has a choice, that choice is useful in determining whether the US has good medical care.
So, there are two ways to think of C:
1) people who have a choice pick US medical care: strengthens the argument; or
2) people who have a choice pick US medical care, but what does that have to do with a vaccine lab's relationship to protection from infection diseases?: outside the scope.
Regardless of whether it strengthens or is outside the scope, it certainly doesn't weaken: pick C.
Last edited by Stuart@KaplanGMAT on Thu Nov 26, 2009 1:09 pm, edited 1 time in total.
Stuart Kovinsky | Kaplan GMAT Faculty | Toronto
Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
. This question took me 3 minutes to answer. YES 
...
