551.When do we need to pay the balance?
________ September 30?

A.In

B.By

C.During

D.Within

552.People have always been curious _____ how living thigns on the earth exactly began.

A.in

B.at

C.of

D.about

551.答案是B;

by 不迟于，在…之前；the balance 余欠之款。

552.答案是D;

be curious about sth.对某事物感到好奇。