第1个回答 2007-03-01
We must protect the environment.
第2个回答 2007-03-01
We should protect the environment
第3个回答 2007-03-01
We must protect our environment.
第4个回答 2007-03-01
we ought to \must protect the enviroment.
第5个回答 2007-03-01
We must protect the environment