正常成人右下肺动脉主干内径一般不超过（）
A.5mm
B.10mm
C.15mm
D.20mm
E.25mm
参考答案：C
从圆锥的______到______的距离是圆锥的高，圆锥有______
A．顶点 B．底面圆心 C．一条高 D．无数条．
It's dangerous to play on the thin ice. It's not ____ to hold a person. [ ]
A. hard enough
B. enough hard
C. enough cold
D. enough thick
