A Toroidal Solenoid Has a Central Radius of 0

Multiple Choice

A toroidal solenoid has a central radius of 0.50 m and a cross-sectional diameter of 10 cm.When a current passes through the coil of the solenoid,the magnetic field inside the solenoid at its CENTER has a magnitude of 2.0 μT.What is the largest value of the magnetic field inside the solenoid when this current is flowing? (μ₀ = 4π × 10^-7 T • m/A)

A) 3.5 µT
B) 1.8 µT
C) 2.2 µT
D) 0.50 µT
E) 2.8 µT