A vehicle is fitted with a microwave transmitter that produces microwaves at a fixed frequency. A stationary observer has a microwave receiver. The vehicle travels straight towards the observer at constant speed. The observer detects microwaves with frequency $F_o$. The vehicle then speeds up, while still moving towards the observer, continues at a larger constant speed for a period and then slows down until it comes to rest. How does the frequency of the microwaves detected by the observer change?
- AThe observed frequency will fall, then remain steady then return to the frequency $F_o$.
- BThe observed frequency will fall, then remain steady then rise to a higher frequency than $F_o$.
- CThe observed frequency will rise, then remain steady then fall to a lower frequency than $F_o$.
- DThe observed frequency will rise, then remain steady then return to the frequency $F_o$.