如图所示,空心圆轴的外径为D,内径为d,且D=2d,其抗扭截面系数为( )。
A <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251103/21572495_0.png"> B
<img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251103/21572495_0.png"> C
<img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251103/21572495_0.png"> D
<img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251103/21572495_0.png">