Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

[EX Q]: Bacteria is usually the prey of a protist.
[EX A]: What is usually the prey of a protist?

[EX Q]: Diffusion process helps absorptive protists absorb food molecules across their cell membranes.
[EX A]: Which process helps absorptive protists absorb food molecules across their cell membranes?

[EX Q]: Because all animals require an external source of food, they are called heterotrophic.
[EX A]:
Because all animals require an external source of what, they are called heterotrophic?