问题补充：
Drivers must stop when the traffic lights red．
A. turn
B. will turn
C. turned
D. were turning
答案：
(答案→)A. turn
B. will turn
C. turned
D. were turning
解析：当红绿灯变绿的时候司机必须停下来。When引导的时间状语从句的时候，用一般现在时代替一般将来时，所以选A。
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