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19. A radio-wave transmitter and a receiver are separated by a horizontal distance $d = 60.0 \, \text{m}$. Both antennas are located at a height $h = 40.0 \, \text{m}$ above the ground, as shown in the figure. The receiver detects two signals: one travels directly from the transmitter, and the other is reflected from the ground. Assume the ground is perfectly level and that a $180^{\circ}$ phase shift occurs upon reflection. What is the longest wavelength that results in destructive interference at the receiver?
(A) $20.0\mathrm{m}$
(B) $26.7\mathrm{m}$
(C) $40\mathrm{m}$
(D) $80\mathrm{m}$
(E) $160\mathrm{m}$
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統計： 尚無統計資料
統計： 尚無統計資料
