A driver moves toward a stationary hill while sounding the horn. The frequency of the echo heard by the driver is higher than the emitted frequency mainly because

First, the hill receives sound from an approaching source, so the frequency at the hill is Doppler shifted upward. Then the reflected sound is heard by the approaching driver, producing another upward Doppler shift. Thus the echo frequency is affected twice, making A correct. Speed of sound in the medium does not increase because of reflection, so B is false.