[题目]-|||-边长为l的正方形线图中通有电流I,此线圈在A点-|||-(见图)产生的磁感强度B为 ()-|||-A-|||-A. dfrac (sqrt {2)(mu )_(0)I}(4pi L) .-|||-B. dfrac (sqrt {2)(mu )_(0)I}(2pi L)-|||-C. dfrac (sqrt {2)(mu )_(0)I}(pi l)-|||-D.以上均不对