Given a scientific question, generate a correct answer to it.

[EX Q]: What is the term for the movement of molecules from an area of high concentration to an area of low concentration?
[EX A]: diffusion

[EX Q]: Bacteria, viruses, fungi, and protozoa are examples of what, the term for a disease-causing microorganism?
[EX A]: pathogen

[EX Q]: Covalent solids are formed by networks or chains of atoms or molecules held together by what?
[EX A]:
covalent bonds