It’s very important for a student to ________ a good state of mind while taking the college entrance examination.

A．keep out

B．keep on

C．keep up

D．keep off

一般情况下，材料的孔隙率小且连通孔隙少时，其下列性质中表述不正确的是（）。

A.强度较高

B.吸水率小

C.抗渗性好

D.抗冻性差