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

 Problem 5. 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 [NEET 2007]

(a) $\sqrt{v_u v_d}$

(b) $\frac{v_d v_u}{v_d + v_u}$

(c) $\frac{v_d + v_u}{2}$

(d) $\frac{2 v_d v_u}{v_d + v_u}$