Two long solenoids S1 and S2 have equal lengths and the solenoid S1 is placed co-axially inside the solenoid S2. The mutual inductance of both the solenoids is proportional to the:
1
Square of the radius of the inner solenoid
2
Square of the radius of the outer solenoid
3
Radius of the inner solenoid
4
Radius of the outer solenoid