If you like to work outside,you could help ______ city park.

问题描述:

If you like to work outside,you could help ______ city park.
A.clean up
B.sweep put
C.clear up
D.to clean up
我知道课本上的原话是A,但是为什么不是D呢?

help do sth 固定结构我记得有help sb to do sth.