A cyclist pedals his bike and reaches the top of a hill. Then he begins to coast downhill without pedaling. When he reaches the bottom of the hill, he again applies force to the pedals.

Why does the cyclist accelerate as he coasts downhill?

Question

A cyclist pedals his bike and reaches the top of a hill. Then he begins to coast downhill without pedaling. When he reaches the bottom of the hill, he again applies force to the pedals. 
 
Why does the cyclist accelerate as he coasts downhill?

anonymous · Answer

A.
The force of friction propels the cyclist and the bicycle down the hill.
 
 B.
The mass of the cyclist opposes the mass of the bicycle, causing it to accelerate downhill.
 
 C.
The unbalanced force of gravity acts on the mass of his body and the bicycle.
 
 D.
The force of the cyclist and the force of the bicycle added together increase the downhil