Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is -

Question

( A ) $\sqrt{\frac{f_1}{f_2}}$
( B ) $\frac{f_1+f_2}{f_1 f_2}$
( C ) $\sqrt{\frac{f_2}{f_1}}$
( D ) $\frac{f_1+f_2}{2}$