A toroid has a non-ferromagnetic core of inner radius 25 cm and outer radius 26 cm around which 3500 turns of a wire are wound. What is the inductance?
a) 2π × 10⁻⁴ H
b) 5π × 10⁻³ H
c) 2.5π × 10⁻³ H
d) 7π × 10⁻⁴ H