---There is somebody knocking at the door. Who________ it be?

---I’m not sure. It ________ be a postman.

A．may; must

B．will; can

C．can; may

D．may; will

KY-310型牙轮钻机钻架四根立柱采用的是（）。

A.方钢管

B.等边角钢

C.不等边角钢

D.槽钢