Two long ideal solenoids (with radii 20 mm and 30 mm, respectively) have the same number of turns of wire per unit length. The solenoid is mounted inside the larger, along a common axis. The magnetic field with in the inner solenoid is zero. The current in the inner solenoid must be:

a. two-thirds the current in the outer solenoid
b. one-third the current in the outer solenoid
c. twice the current in the outer solenoid
d. half of the current in the outer solenoid
e. the same as the current in the outer solenoid