If the second part is a clause, where is the verb?

why is D wrong????

A is wrong because the second clause is a fragement.

D is wrong because it refers to to syntetic drugs while it shouldn't.

amaelle wrote:A is wrong because the second clause is a fragement.

D is wrong because it refers to to syntetic drugs while it shouldn't.

D is wrong not because 'it' mistakenly refers to synthetic drugs. In fact, 'it' should refer to synthetic drugs (growth hormone), BUT needs revision as 'it' may also refer to gerund form used 'learning'
Choice D is not quite logical 'to make synthetic' - can the synthetic be not made? Better use 'to synthesize' instead of 'to make synthetic'

amaelle wrote:Before scientists learned how to make a synthetic growth hormone, removing it painstakingly in small amounts from the pituitary glands of human cadevers.

A)
B) scientists had learned about making a synthetic growth hormone, they had to remove it painstakingly.
C) scientists learned how to synthesize the growth hormone, it had to be painstakingly removed
D) learning how to make a sythetic growth hormone, scientists had to remove it painstakingly
E) learning how to synthesize the growth hormone, it had to be painstakingly removed by scientists

...

In choice A agree is fragment, choices B,D contain 'to make synthetic' and may be crossed out, in choice E the second clause starts with 'it' and signals performing action by 'it' -> 'learning how to ...'. Choice C is only correct after crossing out the other choices. Choice C could be grammatically not 100% correct, BUT is correct here. To be 100% grammatically correct choice C should have been stated in the proper sequence of tenses -> C) scientists learned how to synthesize the growth hormone, it had had to be painstakingly removed.

amaelle wrote: my question is the following:
the OA is C but according to my understanding we have two related clauses, the growth hormone is the object of the first clause, and it is a subject pronoun that refer to growth hormone in the second clause. i thought that objects should be referred to in the second clause by object pronouns?
please correct me if i'm missing something.
thanks a lot in advance

'it' functions as both forms of pronoun - subject and object.

Op A, B, D are logically incorrect. These options suggest that scientists remove "sythetic growth hormone" from the "pituitary glands of human cadevers"....this is nonsensical....as synthetic thing can not be removed from a human body, a human body does not create a synthetic thing and so the correct logic is scientist created the synthetic growth hormone

Op E has modifier issue..."before learning...." must be followed by "scientists (who learning)", hence wrong

left with OP C the correct answer