# Two coaxial solenoids are made by winding thin insulated wire over a pipe of cross sectional area $A = 10 cm^2$ and length = 20 cm. If one of the solenoids has 300 turns and the other 400 turns, their mutual inductance is ($\mu_0= 4\pi \times 10^{-7} Tm A^{-1}$)
( A ) $4.8\pi\times10^{-4}H$
( B ) $2.4\pi\times10^{-4}H$
( C ) $2.4\pi\times10^{-5}H$
( D ) $4.8\pi\times10^{-5}H$