It was not until the early 1950s most tape recorders began using plastic tape as they do t

It was not until the early 1950s most tape recorders began using plastic tape as they do today.
A. when
B. which
C. that
D. Before

A. whenB. which(答案→)C. thatD. Before解析：考查强调句的用法。根据句子结构可知，此句为强调句，强调not until the early 1950s, 完整的句子结构为Most tape recorders didn't begin using plastic tape as they do today until the early 1950s.

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