I think so it ans wold be d.because if the source approaches the observer more and more wavefront will pass and it get squeezed.so wavelength decreases and frequency increases.
        
                    
             
        
        
        
Answer:
If we are looking for evidence of something that exists outside of our visible Universe and leaves no trace within it, it seems that the idea of a Multiverse is fundamentally untestable.  But there are all sorts of things that we cannot observe that we know must be true. Decades before we directly detected gravitational waves, we knew that they must exist, because we observed their effects.
Explanation:
Maybe helps lol
 
        
                    
             
        
        
        
Answer:
The true course:  north of east
 north of east
The ground speed of the plane: 96.68 m/s
Explanation:
Given:
 = velocity of wind = = velocity of wind = 
 = velocity of plane in still air = = velocity of plane in still air = 
Assume:
 = resultant velocity of the plane = resultant velocity of the plane
 = direction of the plane with the east = direction of the plane with the east
Since the resultant is the vector addition of all the vectors. So, the resultant velocity of the plane will be the vector sum of the wind velocity and the plane velocity in still air.

Let us find the direction of this resultant velocity with respect to east direction:

This means the the true course of the plane is in the direction of  north of east.
 north of east.
The ground speed will be the magnitude of the resultant velocity of the plane.

Hence, the ground speed of the plane is 96.68 km/h.