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A car travels at a constant speed along a straight line PQ. A loudspeaker attached to the car emits sound of constant frequency 
. A stationary observer is at point O.
What does the observer hear as the car moves from P towards Q?
a frequency less than that decreases as the car moves from P towards Q
a frequency less than that increases as the car moves from P towards Q
a frequency more than that decreases as the car moves from P towards Q
a frequency more than that increases as the car moves from P towards Q
