A sound source with frequency $F$ at point $Z$ is travelling at a constant speed. The arrangement of the wavefronts it emits is shown. Which row gives the frequencies of the sound detected by stationary observers at $X$ and $Y$?
- Afrequency heard at $X$ is $< F$, frequency heard at $Y$ is $< F$
- Bfrequency heard at $X$ is $< F$, frequency heard at $Y$ is $> F$
- Cfrequency heard at $X$ is $> F$, frequency heard at $Y$ is $< F$
- Dfrequency heard at $X$ is $> F$, frequency heard at $Y$ is $> F$