从A、B、C、D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
21．Keeping a diary is ____ good habit, by which you'll make ____ great progress in writing.
A．a; a B．a; 不填 C．the; a D．the; 不填
22．—Excuse me, is this shop open all day?
—____．It's only open from 9 a．m．to midnight．
A．Well, I'm not sure B．Oh, it's nothing special
C．Sorry, I'm afraid not D．Yes, of course not
23．—We've just sold out the last copy of this dictionary．
—Well, it doesn't mailer．I think I might get ____ in another bookshop.
A．one B．that C．this D．it
24．It was not until the couple arrived at the cinema ____ they found they had left the tickets in the office．
A．when B．where C．which
