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