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: Heat is necessary for the survival of animals. 
Question: What is necessary for the survival of animals?

Answer: heat.


Question: Fact: navigation requires knowing which way to go. 
Question: What requires knowing which way to go?

Answer: navigation.


Question: Fact: Clams feed on plankton and nonliving organic matter. 
Question: What do clams feed on?

Answer:
Plankton.