GMAT Sentence Correction (SC) | Re: By the late 1960s, the rising number

You are mistaken in that a past perfect always requires another verb or event. A time reference may also require a past perfect tense.By + time reference MUST take a past perfect as explained in my post above:

by-the-late-1960s-the-rising-number-187705.html#p1783223

The
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