21.B
however convincing = no matter how convincing
22.A
shouldn't have been doing sth 本不该(正在)做某事（但事实上当时正在做）
23.C
now当然一般是用现在进行时
24.C
if引导的条件状语从句如果是对过去发生动作的虚拟,则从句用过去完成时,主句用would/could/might have done sth；
且此题句意为被动
25.C
固定句型：no sooner...than...一...就...
26.C
I'm的反意疑问句,一般用aren’t I
27.C
肯定句中只能用as...as...
as high as one thousand dollars a semester 学费高达每学期一千美元
28.B
以I wish开头的句子的反意疑问句,一般用may I.
29.D
“I think that从句”中,如果从句是否定句,则一般出现否定前移现象,即从句不否定,I think变成I don't think
30.A
dare没用三单形式,则表明dare是情态动词,所以其反意疑问句用dare来变