287.The writer of the novel is a professor in his ______.

A.thirtieth

B.thirty

C.thirty's

D.thirties

288.How many pens would you like,Madame?

Well,I want _____ these.

A.three dozen

B.three dozen of

C.three dozens

D.three dozens of

287.答案是D；

in one's thirties 在某人三十多岁时。

288.答案是B;

dozen前有基数词时，其后一般不与of连用；但如果名词前有these,those,the,one's等时，须接of。如:three dozen pens,three dozen of the pens。