31 一端開口，一端封閉的管狀構造，其最低的共振頻率 F（lowest resonant frequency）為（c = 聲音速度， L = 管子長度）
(A) F = c/L
(B)F = c/2L
(C)F = c/4L
(D)F = c/8L

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統計： A(1), B(9), C(45), D(1), E(0) #1077588