The core of a solenoid is made of material of relative permeability μr. A small cavity area dA and length dl is cut in the core. If it is desired that flux density in the cavity should be equal to B0

• the fields inside and outside the cavity must be equal
• the field inside the cavity must be made stronger than field outside the cavity
• the field inside the cavity must be made weaker than the field outside the cavity
• none of the above