Question $43(2$ points$)$

Listen

Doppler effect is

the heard or observed frequency due to motion of waves.

the change in the heard or observed frequency because of the motion of the source such as an approaching or receding ambulance with its siren on$.$

the change in the heard or observed frequency because of the motion of the observer such as driving toward or away from an ambulance that has its siren on$.$

b and c are both correct.