Scientists are convinced ______ thepositive effect of laughter ______ physical and mental health.

下列各项中，划线的成语使用有误的一项是（ ）

volleyball is this—It must belong to Grace. She loves volleyball.

Listen! There is a knock on the door.Who _____ it be?—It must be Jack.Just now I sent for him. [ ]

He used to _____ games on the Internet, but now he gets used to ______ a walk after supper.