A car moves from X to Y with a uniform speed $v_u$ and returns to Y with a uniform speed $v_d$. The average speed for this round trip is:-

Question

( A ) $\frac{v_d v_u}{v_d+v_u}$
( B ) $\sqrt{v_u v_d}$
( C ) $\frac{v_u + v_d}{2}$
( D ) $\frac{2v_dv_u}{v_d+v_u}$