— Mary, whose photo is it?—It ______ be Lu’s. Those are her parents.

—_____ I finish the book this week?—No, you ____ . You ____ return it to me the week after next

Sometimes smiles ______ be false, hiding other feelings like anger, fear or worry.

 —I’ll call Bob and tell him the good news.—You __________ do that. He was here just now and heard it.

Trees ____ fight air pollution．They are natural air conditioners．