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

Input: Consider Input: Bacteria is usually the prey of a protist.

Output: What is usually the prey of a protist?

Input: Consider Input: Together, the muscular and skeletal organs are known as the musculoskeletal system.

Output: Together, the muscular and skeletal organs are known as what?

Input: Consider Input: The number of particles of a substance in a given volume is known as the concentration.
Output: The number of particles of a substance in a given volume is known as?