[单选题]
A car of speed $3\,v$ approaches another car of speed $v$ from the rear on a level road.
The car with a speed of $3v$ uses its horn to clear its way. The frequency of the emitted sound wave is $f$ and the speed of the sound wave is $10v$. What is the observed frequency of the sound wave by an observer in the front car?
A. $\frac{2}{3}$
B. $\frac{11}{7}$
C. $\frac{9}{7}$
D. $\frac{10}{7}$
