A source ‘S’ of sound wave of fixed frequency ‘f’ and an observer ‘O’ are located in air initially at the space point A and B, a fixed distance apart. State in which of the following cases, the observer will not see any Doppler effect and will receive the same frequency ‘f’ as produced by the source.

The source ‘S’ remains stationary but the observer ‘O’ and the wind have same speed away from the source.

Both the source ‘S’ and observer ‘O’ remains stationary but wind blows with constant speed in arbitrary direction.

The observer ‘O’ remains stationary but the source ‘S’ moves parallel to and in the same direction and with the same speed as wind.

The source ‘S’ and the observer ‘O’ move directly against the wind but both with the same speed.