— It is so late. Why must you ring him up now?

— Oh, sorry. I ____ it. [ ]

A. don't notice

B. didn't notice

C. haven't noticed

D. won't notice

空冷塔液位的测量，实际上是测量液体上边和液体底部的（）。

A.压力差

B.温度差

C.比容差

D.密度差