An emergency vehicle is emitting its siren while it speeds up on a straight road between points $X$ and $Y$, as shown in the diagram. The siren produces a sound frequency of $750\,\text{Hz}$. One person is positioned at $X$ and another person is positioned at $Y$. What describes the sounds heard by the people at $X$ and at $Y$ as the vehicle accelerates?
- Asound heard by person at $X$: higher than $750\,\text{Hz}$, increasing in frequency; sound heard by person at $Y$: lower than $750\,\text{Hz}$, decreasing in frequency
- Bsound heard by person at $X$: higher than $750\,\text{Hz}$, decreasing in frequency; sound heard by person at $Y$: lower than $750\,\text{Hz}$, increasing in frequency
- Csound heard by person at $X$: lower than $750\,\text{Hz}$, decreasing in frequency; sound heard by person at $Y$: higher than $750\,\text{Hz}$, increasing in frequency
- Dsound heard by person at $X$: lower than $750\,\text{Hz}$, increasing in frequency; sound heard by person at $Y$: higher than $750\,\text{Hz}$, decreasing in frequency