问题补充：
While we were running on the playground,Jack suddenly stopped and lay on the ground,so we all stopped what was wrong with him.
A. running; to see
B. running; seeing
C. to run; to see
D. to run; seeing
答案：
(答案→)A. running; to see
B. running; seeing
C. to run; to see
D. to run; seeing
解析：stop doing sth，停止做某事，stop to do sth停下来去做某事
