如图13所示,无限长直导线在P处弯成半径为R的圆,当通以电流I时,则在圆心O点的磁感强度大小等于:( )<img src="/lqwvjeImg/34/34b9e336d14e2d41cf2ef6b6b2716453.png" style="font-size: 20px;" ><br>·<img src="/lqwvjeImg/18/18d10c2ff84b6f0a238db5fb8363fe15.png"style="width:29px;height:34px;">.<br>·<img src="/lqwvjeImg/e0/e055a159a4417d93286663494e760fc7.png"style="width:26px;height:36px;">.<br>·<img src="/lqwvjeImg/63/63a05f13ec9450b7f6bb7899b10c8b43.png"style="width:66px;height:34px;">.<br>·<img src="/lqwvjeImg/26/2642e09b3afdb8e763df6822507e7ce2.png"style="width:58px;height:33px;">.