A bicyclist encounters a series of hills. Uphill speed is always `v_1`, and downhill speed is always `v_2`. The total distance travelled is `l`, with uphill and downhill portions of equal length. The cyclist's average speed is

Options

A. `v_1/v_2`
B. `v_2/v_1`
C. `(v_1v_2)/(v_1+v_2)`
D. `(2v_1v_2)/(v_1+v_2)`

Answer

D