You are presented with the quiz "What moving downhill due to the force of gravity can dissolve carbon in sedimentary rock?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "flowing water can slowly dissolve carbon in sedimentary rock" and "water falls from the sky and flows downhill due to the force of gravity". 

So, what's the best answer to the question?


 

     water      