单选题 --What's the matter? Why ______
--The signal is against us.
A. the train stopping B. does the train stop
C. is the train stopping D. did the train stop

【正确答案】 C
【答案解析】用排除法。A、B、D都不对，D是过去时，而前面是现在时，所以后面为将来时态更合适。