Sound Waves and the Doppler Effect (4D) Practice Test

A bat emits a call and listens to the echo from an insect flying directly away from the bat. Assume the air is still and the bat is stationary. Which outcome is most consistent with the Doppler Effect for the echo received by the bat relative to the emitted call?

In a lab, a speaker emitting a steady tone rolls on a cart past a stationary microphone. As the cart approaches the microphone, the measured frequency is higher than the emitted frequency. Immediately after the cart passes and begins receding, which statement best reflects the behavior of the detected sound wave?