内分布,电流为I,规定垂直纸面向里为正,-|||-则在点O的磁感强度为 ()-|||-R-|||-O-|||----|||-A .dfrac ({mu )_(0)I}(2R)-dfrac ({mu )_(0)I}(2pi R)-|||-B dfrac ({{u)_(0)}^I}(8R)-|||-C dfrac ({mu )_(0)I}(2pi R)+dfrac (mu d)(4R)