As students, we have classes for early morning till late afternoon．Therefore, take a ten-minute break between class is definitely important, even necessary. Otherwise we may feel both physically or mentally tired. During the ten-minute break we did something to get rid of tiredness．What we need is to have real rest, instead of getting even tired．So don't do anything that will make you too excited．My ten-minute break is always pleased．I usually take some exercise．Sometime I have a free chat with my classmates．When the new class begins, I feel freshly again.

在进行室内渗透试验测定土的渗透系数时，下列( )适合于变水头试验。

A．黏性土

B．砂土

C．碎石土

D．粉土