Recently, however, scientists have been finding that long-term use of these fertilizers can cause damage to the land, thus _____it less fertile.

Who would you like to have ______ you

What is Tom up to? He’s seldom turned in his assignments these days, ________?[ ]

I had a really hard time _____studyingscience this term.

Joe has been studying Chinese for three months．but he can ________ understand people when they speak Chinese．