_______ necessary for us _______ our environment clean and tidy.

the college entrance exam is drawing near, study shouldn’t take up all of your time. It’s important to some time for relaxation and hobbies as well.

If you had told me in advance, I ___ him at theairport.

There will be less pollution _____ more people use public transportation.

Li Ming is my good friend. He often helps me myEnglish.