—Will you be ______ this evening, Mary?—That depends. I am afraid I will be with my manager discussing a plan.

He is always ready to help others. That's one of his ________ .

The hotline_________ be very busy，but keep trying and you’ll get through．

Carbon dioxide, which makes a ______ between us and the sun, prevents heat from getting out of the atmosphere easily, so the earth is becoming warmer.

Parks are _____ of the few places where people in cities may have sports.