问题补充：
It was not until 1920regular radio broadcast began.
A. while
B. which
C. that
D. since
答案：
A. while
B. which
(答案→)C. that
D. since
解析：众所周知，当时间状语由until/till引出时，如果谓语动词为非延续性动词，则要用否定式，表示“直到……才……”。本句的正常结构应为：Regular radio broadcast didn’t begin until 1920.如果强调本句的时间状语，则要把not和until...一起放在句首，可以构成：Not until 1920 did regular radio broadcast begin.（注意表示否定意义的副词位于句首，要倒装），如果使用强调结构，则可构成：It was not until 1920 that regular radio broadcast began.
