如图所示，有一根直导线上通以恒定电流I，方向垂直纸面向内，且和匀强磁场B垂直，则在图中圆周上，磁感应强度数值最大的点是
A．a点
B．b点
C．c点
D．d点
答案：A
题目分析：根据安培定则可知通电导线在a、b、c、d四点处磁场的方向，导线在四点处的磁感应强度大小相同，如下图所示：
根据平行四边形法则可知，与匀强磁场方向相同的a点的磁感应强度最大，所以只有A正确；
| 短文填空。用方框中所给词的适当形式填空。 | |
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