Official ExplanationRead the Original Sentence Carefully, Looking for Errors:
This sentence does not contain any actual grammar errors. However, it is extremely wordy, with the passive "It was not long after ... that" and redundancy such as "1960s decade." Look for an answer choice that is clearer and more concise but that also does not introduce an error.
Scan and Group the Answer Choices:
Like (A), (E) begins with the awkward "It was ... that." The other answer choices begin in various ways. (A), (C), and (D) end in "contributed to," while (B) and (E) end with the idiomatically incorrect "contributed in."
Eliminate Choices Until Only One Remains:
(E) does not improve on the original sentence. In addition, it changes the verb tense at the end to the past perfect "had contributed," which is only properly used to place a past action before another past action. These teachers made their contribution at the same time as the 1960s commenced. Finally, as noted, "contributed in" is not correct. This choice is a loser—eliminate it.
(B) has the same redundancy as (A): the 1960s" is a decade, so there is no need to use the words "the decade" before it. (B) also has the incorrect "contributed in" at the end.
(C) presents another redundancy: the word "beginning" has the same meaning as "commencement" and is therefore unnecessary. Also, like (E), this choice uses the past perfect "had contributed."
Only (D) simplifies the underlined portion and expresses the ideas clearly and correctly.