–Look! The boy is running fast! –He might to catch a bus.A. runB. runningC. is runningD.

问题补充：

–Look! The boy is running fast! –He might to catch a bus.

A. run

B. running

C. is running

D. be running

答案：

A. run

B. running

C. is running

(答案→)D. be running

解析：句意：看，那个男孩跑的真快。他可能在赶公共汽车。根据题干中The boy is running fast可知此处是两人在推测正在发生的事，故用情态动词+be+-ing ，表示对正在发生的事情的推测。选D。