Write a correct answer to the given question based on its associated fact. Make sure that your answer is contained in the associated fact. Don't be creative and introduce any new word that is not mentioned in the associated fact! Remember that the associated fact has been rearranged to form the question. So, the correct answer words must lie within the associated fact. The correct answer can be a word, phrase, or even a sentence.

Fact: Heating with coal can cause air pollution. 
Question: Heating with coal can cause?
air pollution.

Fact: rivers can slowly dissolve carbon in sedimentary rock. 
Question: what can slowly dissolve carbon in sedimentary rock?
rivers.

Fact: Canyons reveal the geologic history of the area. 
Question: Where can one readily see the geologic history of an area?

canyon.