A fixed source of sound emitting a certain frequency appears as `f_(a)` when the observer is apporoaching the source with `upsilon_(0)` and `f_(r)` when the observer recedes from the source with the same speed. The frequency of source is

A. `(f_(r) + f_(a))/(2)`

B. `(f_(a) - f_(r))/(2)`

C. `sqrt(f_(a)f_(r))`

D. `(2f_(r)f_(a))/(f_(r) + f_(a))`