15. The motion of a particle connected to a spring is described by x = 10 sin πt. At what time (in s) is the potential energy equal to the kinetic energy?
(A) 0
(B) 0.25
(C) 0.5
(D) 0.86
(E) 1.0

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統計： A(0), B(7), C(1), D(2), E(1) #3224661