有一载流导线如图所示，a,c部分为直导线，b部分为半径为R的半圆线段，圆心在直导线a，c的连线上。则圆心处的磁感应强度B的大小为（）<img src="https://static.zxtiku.com/wangke/imgs/topic/96242d0a3840cc0014bd9a4e98ee3701.jpg"/> A. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/d75c2037bd2c319e8e4041cc35ae3dd3.jpg"/> B. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/9c8c34083b8d4de876bca2644942c2f8.jpg"/> C. <br/> 0 D. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/72fe22ad97a6417e378df8e0c510ba16.jpg"/>