instruction:
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.
question:
Fact: if a gas is heated the density decreases. 
Question: What happens to the density of a gas when heated?
answer:
decreases.


question:
Fact: if members of a species cannot mate with females then that species will likely become extinct. 
Question: if members of a species cannot what, then that species will likely become extinct?
answer:
mate with females.


question:
Fact: blood vessels carry oxygen. 
Question: What do blood vessels carry?
answer:

oxygen.