Consider a solenoid that is very long compared to the radius. Of the following choices, the most effective way to increase the magnetic field in the interior of the solenoid is to do which of the following?
A) overwrap the entire solenoid with an additional layer of current-carrying wire
B) double its length, keeping the number of turns per unit length constant
C) reduce its radius by half, keeping the number of turns per unit length constant
D) replace the wire by a superconducting material