A clinical trial clinic in California is offering

I think the issue is with parallelism of verbs.

A) leading...slow down - Not parallel.
B) make..slowing down - Not parallel
C) lead - slow - Parallel - Correct
D) lead - slowing - Not parallel
E) increases - Should be increase as it's referring to a plural word - 'transfusions'.

Waiting for the OA.

Cheers!

Nice Question ..
Parallelism + comparison

Will go with C here ...

A clinical trial clinic in California is offering older patients the chance to pay $10,000 to receive blood transfusions from younger people aged 16 to 25; however, scientists say that such transfusions <u>have a much greater likelihood of leading to infections among the recipients than they have to slow down</u> the aging process.
(A) have a much greater likelihood of leading to infections among the recipients than they have to slow down. (Parallelism error)
(B) make infections among the recipients much more likely than a slowing down of-(make infections not correct)
(C) are much more likely to lead to infections among the recipients than they are to slow down( looks correct except I have a doubt on much more)
(D) make it more likely to lead to infections among the recipients than slowing(no antecedent for it)
(E) increases the likelihood of infections among the recipients while slowing down( while changes the meaning)
So answer should be C.

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what does the word they in option C refer to? I thought it ambiguous either refers to recipients or transfusions.
I am a little bit confused with: they are to slow down the aging process.

That they refers to such transfusions, and it is reasonably clear in its reference. I say "reasonably" because there is no way to get to a definite yes/no in this kind of situation. In fact, there are plenty of GMAT questions in which the correct option contains ambiguous pronouns.

Generally, you should focus on the more important concepts before taking a call based on pronoun ambiguity. Ambiguity is not necessarily a deal-breaker.

(A) have a much greater likelihood of leading to infections among the recipients than they have to slow down
Parallelism issue. The structure ... of leading.... is NOT PARALLEL to ... to slow down.... INCORRECT.

(B) make infections among the recipients much more likely than a slowing down of
Meaning Issue : the transfusions are not themselves causing infections instead they are leading to infections. Also, there is parallelism issue. INCORRECT.

The sentence never talks about the REASON for this increased likelihood. It may be because of transfusion themselves or the relatively new procedure or low skill level of doctors or poor after-surgery care.

Option B explicitly states that the procedure itself is the REASON for the increased likelihood. This is not the intended meaning of the sentence.

(C) are much more likely to lead to infections among the recipients than they are to slow down
Here the intended meaning is clear and the parallelism is also perfect. CORRECT.

(D) make it more likely to lead to infections among the recipients than slowing
Here, also there is a parallelism issue. ALso, what does "it" refers to here? There is no logical antecedent for it to refer.

(E) increases the likelihood of infections among the recipients while slowing down
Usage of while is incorrect here, the two actions are not taking place at the same time or are not simultaneous. INCORRECT.

ALSO, the verb form (or adverb form) LIKELY is preferred to the noun form LIKELIHOOD.

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