1. The problem statement, all variables and given/known data Two long ideal solenoids (with radii 20 mm and 30 mm respectively) carry the same number of turnes of wire per unit length. The smaller solenoid is mounted inside the larger, along a common axis. It is observed that the magnetic field within 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 the current in the outer solenoid E. the same as the current in the outer solenoid This one is hard. If you could please provide an explanation with the answer so I could better understand this. THanks in advance. 2. Relevant equations n/a 3. The attempt at a solution Right now I'm ruling out answer A.