如图所示，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="https://nimg.ppkao.com/2019-05/wangjing/2019050916192593998.jpg?sign=098480ce177f281d1726c88d5a3a8806&t=62d6e850" /><img src="https://nimg.ppkao.com/2019-05/wangjing/2019050916193084614.jpg?sign=de5bec47382870d9f0a50c2c5516f487&t=62d6e850" />