A coil of self-inductance `L` is placed in an external magnetic field (no current flows in the coil). The total magnetic flux linked with the coil is `varphi`. The magnetic field energy stored in the coil is
A. zero
B. `(varphi^(2))/(2L)`
C. `(varphi^(2))/L`
D. `(2varphi^(2))/L`
