The car moves at constant speed in a straight line PQ. A loudspeaker fixed to the car emits sound with constant frequency $f$. A stationary observer is located at point O. What sound does the observer hear while the car travels from P to Q?
- Aa frequency less than $f$ that decreases as the car moves from P towards Q
- Ba frequency less than $f$ that increases as the car moves from P towards Q
- Ca frequency more than $f$ that decreases as the car moves from P towards Q
- Da frequency more than $f$ that increases as the car moves from P towards Q