用所给单词的适当形式填空。
【小题1】Do you know how ____________ (protect) the Earth.
【小题2】What can we do ___________(keep) our city clean?
【小题3】--Look at the sign. You can’t talk here.
--Sorry. We ___________(not see) it just now.
【小题4】He would like____________ (go) shopping with you.
【小题5】What __________（make）the river dirty?
【小题6】Well ________(do), children!

I can learn ________ ping­pong. (    )

A．to play

B．plays

C．playing

They decided _________ a spaceship together. (   )

A．make

B．made

C．to make

It’s sunny. I can’t wait _______ outside to play. （   ）

A．go

B．go to

C．to go

It is difficult _______ English well. (  )

A．learning

B．learns

C．to learn