It was not rare(稀少) in that people in fifties were going to school for further study.A. 90

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A. 90s; the
B. the 90s; /
C. 90s; their
(答案→)D. the 90s; their
解析：考查词组：第一空考查in the+年代的复数或加’s,是在多少世纪多少年代，第二空考查in one’s 数词的复数，表示“在某人多少岁的时候”，句意：在90年代，很少有人在50几岁的时候还去上学深造的。选D。