4.空心圆轴的外径为D，内径为d,a=d/D.其抗扭截面系数为
A Wp=πD^3/16(1-a)
B Wp=πD^3/16(1-a^2)
C Wp=πD^3/16(1-a^3)
D Wp=πD^3/16(1-a^4)