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: Phenotypic expression of skin color varies over a wide range. 
Question: Phenotypic what of skin color varies over a wide range?

Answer: expression.


Question: Fact: Genes are molecules that contain a single protein. 
Question: what are molecules that contain a single protein?

Answer: genes.


Question: Fact: Biomass is the total mass of living things at a trophic level. 
Question: Biomass is the total mass of living things at each what?

Answer:
trophic level.