21．He raised his voice, ______ we still couldn't hear anything．
A．so B．or C．but D．as
22．—What about ______ lecture you attended yesterday?
—To tell the truth, it was too boring．I can't stand ______ lecture like that．
A．a; the B．the; a C．the; 不填 D．the; the
23．She tried two different methods, yet ______ of them seemed to work very well．
A．neither B．none C．each D．both
24．—Ring me at six tomorrow morning, will you?
—Why that early? I ____.
A．will be sleeping B．will sleep
C．have slept D．have been sleeping
25．It is still unclear ______ the little boy got the CD player to work．
A．where B．what C．that D．how