直径为的实心圆轴受扭，若使扭转角减小一半，圆轴的直径需变为（　　）。

A、<img width="33" height="23" src="http://admintk.100xuexi.com/UpLoadImage/2016-10-26/adb0ee9e-2b3b-434a-b1f6-2eba206ccacd.png" />

B、<img width="33" height="23" src="http://admintk.100xuexi.com/UpLoadImage/2016-10-26/a5c0c3e1-13ea-404e-bc70-9f842167fe61.png" />

C、<img width="35" height="19" src="http://admintk.100xuexi.com/UpLoadImage/2016-10-26/6eb3dc5c-b6bb-426c-a7e7-4df1874cb4a7.png" />

D、<img width="23" height="19" src="http://admintk.100xuexi.com/UpLoadImage/2016-10-26/d5897e0c-3863-421d-8525-4c10a1f97c49.png" />