Intuition for Change in Shrillness of Sound

Q. An observer approaches towards a stationary source of sound at constant velocity and recedes away at the same speed. The graph of wavelength observed with time is:

1. 
2. 
3. 
4. 

Q. The engine of a train sounds a whistle at frequency $\nu$. The frequency heard by a passenger is (If wind is blowing from passenger side to Train side)

1. $\nu$
2. $<\nu$
3. $>\nu$
4. $\approx \frac{1}{\nu }$

Q. An observer approaches towards a stationary source of sound at constant velocity and recedes away at the same speed. The graph of wavelength observed with time is:

1. 
2. 
3. 
4.