The frequency of the wave has not changed.
In fact, the frequency of a wave is given by:

where v is the wave's speed and  is the wavelength.
 is the wavelength.
Applying the formula:
- In air, the frequency of the wave is:

- underwater, the frequency of the wave is:

So, the frequency has not changed.
 
        
                    
             
        
        
        
D. 980, this is the best answer because 35 x 7 is 980 :)
        
             
        
        
        
Time=50s
speed=25m/s
Distance = speed×time
=25×50
=1250m
DISTANCE TRAVELLED IS =1250m 
 
        
        
        
I think it’s A.) it explains why the universe is made up of matter