A sound wave with frequency $f$ is produced by a source S that is not moving. The source then travels away from an observer who remains stationary. Which statement is correct?
- AThe frequency of the source S and the observed frequency are now both higher than $f$.
- BThe frequency of the source S and the observed frequency are now both lower than $f$.
- CThe frequency of the source S is now lower than $f$.
- DThe observed frequency is now lower than $f$.