He ____ a machine the last time we saw him.

问题描述:

He ____ a machine the last time we saw him.
A.was operating on B.was operating
Why 这2个区别?

operate sth
操纵某物
operate on sth
给……做手术
这样明白了吧?怎么能给机器做手术呢?所以答案是B