The density is determined on the steepness of the slope. The greater the density is bases upon the steepest slope. To conclude, I'd say Line A has the steepest slope therefore has the greatest density.
Atmospheric pressure is molecular's heavy. So the total number of molecules decreases by going up the surface of the earth. Ok?
Total energy =kinetic energy +potential energy
Change in energy =change in (kinetic energy +potential energy)
potential energy, 
where m is the mass, g is acceleration due to gravity and h is the height.
potential energy per unit mass =gh
change in potential energy per unit mass = 
where, h is the height.
kinetic energy= 
change in kinetic energy per unit mass,
In the given question:
Height varies from 90 m to zero as river flows from 90 m height to lake at 0 m
Velocity varies from 3m/s at top to o m/s at bottom.
Therefore,

Here, it was mentioned in the question internal energy of the water is constant and there is no change in the pressure at the inlet and outlet.
Answer:
Explanation: N2 Nitrogen, O2 Oxygen, Ar does not form molecules