.From the classroom window we saw a huge snake the yard.A. crossing B. crossC. acrossD. h

问题补充：

.From the classroom window we saw a huge snake the yard.

A. crossing

B. cross

C. across

D. having crossed

答案：

(答案→)A. crossing

B. cross

C. across

D. having crossed

解析：句意：从教室的窗户我们可以看到一条巨蛇在院子里正在爬过。短语： see sb /sth doing 看到某人或某物正在做某事，强调正在进行的动作。所以选A。