6.A plane flies at 2 times the sound speed. Its sonic boom reaches a man on the ground~9 sec after the plane passes directly directly overhead. overhead What is the altitude (in unit of m) of the plane roughly? The sound speed is about 340 m/s.
(A) 340
(B) 1020
(C) 2040
(D) 2360
(E) 4080

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