A car moves from X to Y with a uniform speed v(u) and returns to Y with a uniform speed

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

(a) √v(u)v(d)
(b) v(d)v(u)/v(d)+v(u)
(c) v(u)+v(d)/2
(d) 2v(d)v(u)/v(d)+v(u)

2v(d)v(u)/v(d)+v(u)

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