As written, the end of this sentence seems to indicate that earlier high-strength varieties of an antibiotic developed strains of bacteria resistant to the antibiotic. You need an answer choice that makes it clear that the two unintended effects, killing of benign bacteria and developing resistant strains, were encountered with the earlier versions of high-strength antibiotics.
Scan and Group the Answer Choices:
Each answer choice rearranges elements of this sentence in a different way, making it difficult to group the answer choices at a glance. Read through the choices systematically and eliminate them as you find errors.
Eliminate Wrong Answer Choices:
(A) is the sentence as written, which as discussed above is incorrect.
(B) uses the unidiomatic "unintended effects by," and (C) uses the likewise unidiomatic "unintended effects for."
(D) has a modification error. It incorrectly states that the bacteria were "required by" the earlier antibiotics. This choice also loses the information that the effects of the earlier antibiotics were "unintentional," changing the meaning of the original sentence.
(E) is the only choice that makes it clear that the two unintended effects were the effects of the earlier antibiotics. It is the correct choice.
TAKEAWAY: When awkward wording makes the meaning of a sentence unclear, look for an answer choice that removes the ambiguity and doesn't introduce any new errors.