21．You were very impolite to him，for________you should make an apology to him，I think.
A．this B．which
C．what D．that
22．I bought an ancient Chinese vase，________was very reasonable.
A．which price
B．the price of which
C．its price
D．the price of whose
23．________has already been pointed out，grammar is not a set of dead rules.
A．As B．It
C．That D．Which
24．Whenever I met him，________was fairly often，I liked his sweet and hopeful smile.
A．what B．that
C．which D．when
25．He made another wonderful discovery，________of great importance to science.
A．which I think is
B．which I think it is
C．which I think it
D．I think which is
