Crayfish are freshwater crustaceans. They breathe through feather-like gills and are found in water that does not freeze to the bottom. They are mostly found in brooks and streams where there is fresh water running. Most crayfish cannot tolerate polluted water. Crayfish feed on living and dead animals and plants. Dennie wanted to see if crayfish preferred one kind of food over another. He gave one group of crayfish plants, both living and dead, to eat. He gave the other group insects, living and dead, to eat. What is the independent variable in Dennie's experiment?

A) type of food
B) weight of crayfish
C) type of water used
D) age of the crayfish