The temperature of the water at the bottom of a waterfall is greater than the temperature of the
water at the top.
The gravitational potential energy of the water at the top is transferred to thermal energy at the
bottom.
The specific heat capacity of water is 4200 J/(kg °C).
What is the temperature difference for a waterfall of height 21 m?
A 0.005 °C
B 0.05°C
C 20°C
D 200°C

The answer is B (0.05C), but how?
Can someone explain?