Which statement is an example of a scientific theory?

a.
the amount that caffeine affects a person's heart rate differs based on the person's age.
b.
hummingbirds do not like the color white because they visit other colored flowers more often.
c.
the earth is the third planet from the sun and has one moon, which orbits the earth.
d.
all living organisms are composed of one or more cells, which came from pre-exis