instruction:
Given a scientific question, generate a correct answer to it.
question:
What are most protist diseases in humans caused by?
answer:
protozoa

question:
Oxygen reaches what veinless part of the eye by diffusing through its tear layer?
answer:
cornea

question:
What is a reaction in which an atom or molecule is added to an unsaturated molecule, making a single product?
answer:
an addition reaction