—Few pupils are as bright as he is, and he works very hard, too.

—It is no          that he always gets the first place.

A．problem

B．evidence

C．wonder

D．doubt

为保证聚合物溶液达到要求，混配水的总矿化度最好在200-400mg/L之间，最高不应超过（）mg/L。

A、400

B、600

C、800

D、900