距离表面的距离为标准渗透深度的点处的涡流密度约降至表面涡流密度的（）。
A.63%
B.50%
C.37%
D.10%
参考答案：C
如图，①和②都跨过关节连在不同的骨上。因此，骨、关节、肌肉关系图正确的是（ ）
A B C D
Jackie Chan was one of the most famous _________ in the world. [ ]
A. actress
B. actresses
C. actor
D. actors
