since the number of students in public schools have grown steadily since the mid-1980's and, at nearly 47 million, are at a record high.
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gmat因因酱2017-11-03 14:49:09
if we start the underline part(like the one in choice A) by : since the number of students in public schools have grown steadily since the mid-1980’s (here the first " since" convey cause-and-effect relationship)but it makes no sense to say that because the number grow → increase is due to higher enrollment(what sounds weird logically)the underlined part should be converted to a portion into further illustrations of “ high enrollment”