21．—Are you going to visit him this afternoon? —Well,____．
A．I don't know B．it depends C．never mind D．it doesn't matter
22． We were swimming in the lake____ suddenly the storm started.
A. before B. while C. until D. when
23. When ___ such a difficult problem, he didn’t know what to do.
A. facing B. faced C. faced to D. facing with
