It was not until 1920 _______ regular radio broadcast began.

A. while
B. which
C. that
D. since

答案C
本题考查两个句型:①not...until...直到……才……;②强调句型It is/was...that...。until引导的时间状语从句作被强调部分时,应把not和until放在一起。
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