A Toroidal Solenoid Has a Radius R = 0

Multiple Choice

A toroidal solenoid has a radius R = 0.5 m and a cross-sectional diameter equal to 10 cm. When a current I passes through the coil of the solenoid, the magnetic field at the center of the solenoid has a magnitude B = 2 μT. What is the largest value of the magnetic field in the solenoid for the same value of the current?

A) 3.5 μT
B) 1.8 μT
C) 2.2 μT
D) 0.5 μT
E) 2.8 μT