Two Solenoids of Equal Length Are Each Made of 2000

Multiple Choice

Two solenoids of equal length are each made of 2000 turns of copper wire per meter. Solenoid I has a 5.00 cm radius; solenoid II a 10.0 cm radius. When equal currents are present in the two solenoids, the ratio of the magnitude of the magnetic field B_I along the axis of solenoid I to the magnitude of the magnetic field B_II along the axis of solenoid II, B_I/B_II, is

A) 1/4.
B) 1/2.
C) 1.
D) 2.
E) 4.