下列各句中,表达得体的一句是

Who helped Li Lei with his English —______, he learnt it by himself.

I did not see your sister at the meeting. If she , she would have met my brother

The water and soil conservation project resulted infarmers ______ the crops on the farmland _____ trees and grass.

ing to a recent survey, homework is ______ students most often blame for their lack of sleep．