21．Everyone here，including kids，________ their attention on the matter.
A．focus B．focuses C．to focus D．focusing
22．—I must apologize for ________ ahead of time.
—that’s all right.
A．letting you not know B．not letting you know
C．letting you know not D．letting not you know
23．Jenny，you ________ play with the knife；you ________ hurt yourself.
A．won’t；can’t B．shouldn’t；must
C．mustn’t；may D．can’t；shouldn’t
