The cross section above shows a long solenoid of length l and radius r consisting of N closely wound turns of wire. When the current in the wire is I, the magnetic field within this solenoid has magnitude Bo. A solenoid with the same number of turns N, length l, and current I, but with radius 2r, would have a magnetic field of magnitude most nearly equal to
(A) Bo/4 (B) Bo/2 (C) Bo (D) 2Bo (E) 4Bo
是说通电螺线管是2R吧 一楼 。。。~.~!