D f/f0 = v/c

D f = (25/340)60kHz = 4.41 kHz

Frequency incident on the pole and reflected is f1 = 60 + 4.41 = 64.41 kHz

Heard by the bat (moving receiver): f2 = 64.41 + 4.41 = 68.8 = 70 kHz

Answer is D

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