it was not until 1920()regular radio broadcast began.

A:while

B:which

C:that

D:since

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答案：C:that

解析：It was not until 1920 that regular radio broadcast began.

直到1920年,才有收音机.

that引导not until的从句,为：It is not until...that...句型,意思是：直到...才...