5. A car of mass m moving at a speed \( v_1 \) collides and couples with the back of a truck of mass 2m moving initially in the same direction as the car at a lower speed \( v_2 \). What is the speed \( v_f \) of the two vehicles immediately after the collision?

(A)\( v_f = \frac{2}{3}(v_1 - 2v_2) \)

(B)\( v_f = \frac{2}{3}(v_1 + v_2) \)

(C)\( v_f = \frac{1}{3}(v_1 - 2v_2) \)

(D)\( v_f = \frac{1}{3}(v_1 + 2v_2) \)

(E)\( v_f = \frac{1}{2}(v_1 + v_2) \)

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