问题补充：
The boy found a wallet on the ground and stopped .
A. picking it up
B. picking up it
C. to pick it up
D. to pick up it
答案：
A. picking it up
B. picking up it
(答案→)C. to pick it up
D. to pick up it
解析：stop to do sth 表示停下去做另一件事，同时，pick ···up为动词+adv的情况，如果其宾语为人称代词时，需位于二者之间，故选C
