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
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
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the change from active --> passive is appropriate here, because the subject of the second part isn't mentioned (or probably even known).
see, the scientists mentioned in the passage are the ones who learned how to synthesize the growth hormone, but they weren't necessarily the same ones who had to remove it. in order to render the second half in the active voice, you'd have to introduce a subject - awkward at best, because who would be the subject? not only that, but the subject is irrelevant: we don't care who had to remove the hormone from cadavers, only that it had to be removed from the cadavers.
remember this:
if the subject of a verb is unknown or irrelevant, use the passive voice.
analogy:
before Marconi and Braun laid the foundation for wireless telegraphy, messages had to be transmitted over wires or carried by hand.
there's no avoiding the passive voice in the second half, without introducing an awkward (and irrelevant) subject, as in 'people had to transmit messages'. that would detract considerably from the quality of the sentence.
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choice e is wrong, because it starts out with an initial modifier lacking a subject; such modifiers MUST modify the noun IMMEDIATELY FOLLOWING the comma. in the case of choice e, this would mean that 'it' (referring to the hormone itself) learned how to make the hormone. that's crazy.
