The road is covered with snow. I can’t understand they want to go to school by motorbike.

问题补充：

The road is covered with snow. I can’t understand they want to go to school by motorbike.

A. why

B. when

C. since

D. that

答案：

(答案→)A. why

B. when

C. since

D. that

解析：考查宾语从句：A. why为什么B. when什么时候C. since自从D. that那么，根据句意：公路上都是雪，我不明白为什么他们想骑摩托车上学。选A。