– Have they found out the of the fire?-- Yes. It was a cigarette end that the fire.A. c

问题补充：

– Have they found out the of the fire?

-- Yes. It was a cigarette end that the fire.

A. cause; started

B. reason; caused

C. causing; caused

D. reason; cause

答案：

(答案→)A. cause; started

B. reason; caused

C. causing; caused

D. reason; cause

解析：第一空考查cause和reason的区别，cause是起因，和of搭配，reason是理由和for搭配，第二空考查start的起火，句意是：--你发现大火的起因吗？--是，是香烟头引起的大火。选A。