21．Refering to reading, we must have the patience to read _____ book so that we will have _________deep understanding of it．
A．the; the B．a; a C．the; a D．a; 不填
22．I stayed there for only two days but ______ felt like two weeks．
A．it B．what C．when D．then
23．______ most of us considered solving the problem as soon as possible, he considered it of no importance．
A．When B．While C．If D．As
24．By next month I __________ the task, then I will go out for holiday with my family．
A．is accomplished B．will accomplish
C．will have accomplished D．have accomplished