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5. A ball A is dropped from the top of a building and at the same time an identical ball B is thrown vertically upward from the ground. When the balls collide the speed of A is twice that of B. At what fraction of the height of the building did the collision occur?
(1) $\frac{1}{3}$
(2) $\frac{2}{3}$
(3) $\frac{5}{3}$
(4) $\frac{7}{3}$