Correct answer choice D : and as a result the welfare program now costs about 3 billion real dollars less to implement than it did

I agree why A, B and C are wrong .But in D , howcome we could use "it did "? "costs" is in present tense and had the later part been in present too , we could have used "does" but since it is in preesnt tense ,we must pass the main verb that is "costed " or "it did cost" . That is what GMAT follows as well .

Could expert please explain.

An example question :
Despite recent increases in sales and cash flow that have propelled automobile companies' common stocks to new highs, several industry analysts expect automakers, in order to conserve cash, to set dividends more conservatively than they were.

A) to set dividends more conservatively than they were
B) to set dividends more conservatively than they have been
C) to be more conservative than they have been in setting dividends
D) that they will be more conservative than they were in setting dividends
E) that they will be more conservative than they have been to set dividends

You can read Ron's explanation for this to learn a lot for this type of construct. His explanation fixed all my doubts about comparison and more .

GMATMadeEasy wrote:Correct answer choice D : and as a result the welfare program now costs about 3 billion real dollars less to implement than it did

I agree why A, B and C are wrong .But in D , howcome we could use "it did "? did we could have used had it been implemented instead of "implement" ? There is nothing that can pass the correct verb for it did .

Could you please explain.

In D, did is standing in for cost:

...the welfare program now costs about 3 billion dollars less to implement than it cost in 1973.

Thanks GMATGuruNY .

My doubt stems from the fact that when in second part tense changes, we MUST use the modified verb form (and auxilliary if applicable) . Could you confirm whether this is right ? so answer choice D should read "it costed " not "it did" .

Could someone clarify the doubt of "it did" please and this is one of important basics used in many SC questions.

GMATMadeEasy wrote:Thanks GMATGuruNY .

My doubt stems from the fact that when in second part tense changes, we MUST use the modified verb form (and auxilliary if applicable) . Could you confirm whether this is right ? so answer choice D should read "it costed " not "it did" .

Sometimes the verb to do is used to replace another verb in order to avoid repetition. Do will be in the same tense as the verb being replaced if the two actions are happening in the same time period:

John runs faster than Mary does.

The verb to do will be in a different tense if the action it represents happens at a different point in time:

The car costs more this year than it did last year.

Does this help?

Thank you for your explanation. I agree with your explanation but I have some examples that make me confuse .

Wrong: "I have never seen an aardvark, but last year my father DID."
Right: "I have never seen an aardvark, but last year my father saw one."

I never saw an aardvard, but my father did . This is correct very likely ?

Could you relate the explanation of this example and yours with a rule to follow ? I am little lost when you can replace it and when you can not ? Though, I am comfortable with the usage if it is in same tense.

Megumi speaks Japanese better than I do. (same tense)
Megumi has visited more countries than I have . (same tense)
Megumi has more skirts than I do.

GMATMadeEasy wrote:Thank you for your explanation. I agree with your explanation but I have some examples that make me confuse .

Wrong: "I have never seen an aardvark, but last year my father DID."
Right: "I have never seen an aardvark, but last year my father saw one."

I never saw an aardvard, but my father did . This is correct very likely ?

Could you relate the explanation of this example and yours with a rule to follow ? I am little lost when you can replace it and when you can not ? Though, I am comfortable with the usage if it is in same tense.

Megumi speaks Japanese better than I do. (same tense)
Megumi has visited more countries than I have . (same tense)
Megumi has more skirts than I do.

You seem to understand correctly.

In order to avoid repetition, we sometimes use do or did to replace verbs in the simple tenses (past, present, future). Be skeptical of an answer choice that uses do or did to replace a verb in one of the more complex tenses (progressive, perfect, etc.).

Hope this helps!

thank you. I just edited a little what I learnt before . It was worth postig my question I believe .

For me the odd thing here is the commencement of the D) and E) choices. How is it possible to use "AND" if we are not connecting the same type of noun (or something acting as a noun)?

Here we have a sequence of actions: something happens AND as a result something else happens. Is this ok?

BastiG wrote:

paes wrote:With adjustments for inflation,the relative cost of welfare benefits has been dropping since 1973, resulting in the welfare program now costing about 3 billion real dollars less to implement than in 1973

(a)resulting in the welfare program now costing about 3 billion real dollars less to implement than
(b)resulting in the welfare program now costing about 3 billion real dollars less to implement as
(c)resulting in the welfare program now costing about 3 billion real dollars less to implement than it did
(d)and as a result the welfare program now costs about 3 billion real dollars less to implement than it did
(e)and the welfare program now costs about 3 billion real dollars less as a result to implement than

[spoiler]OA later
Please explain C and D.[/spoiler]

OA is D.
A explanation why c is wrong:
did should replace the verb but costing isn't really a verb, so C is wrong

But what I don't understand:
I thought that "And" always requires an parallel structure and I can't really see it in D. Can pls somebody comment on that?

Any two independent sentences can be connected with a conjunction. Here, 2 independent sentences are connected with a conjunction"and". The proper format to do so is independent sentence, and independent sentence.
Hope it helps.