晶粒的大小通常由晶粒度等级来表示，标准晶粒度共分为（）。
A.4级
B.7级
C.8级
D.12级
参考答案：C
食管在起始于第_____颈椎水平与下咽部相连，其下端相当于第______胸椎水平与贲门相接。
As the propeller turns，voids are formed on the trailing and leading edges of the propeller blades causing a loss of propulsive efficiency，pitting of the blades，and vibration. These voids are known as（）.
A.advance
B.cavitation
C.edging
D.slip
