An element is made up of
A.
a mixture of different types of atoms.
B.
different types of cells.
C.
only one type of atom.
D.
different types of atoms that have been combined chemically.

C.  

only one type of atom.

Explanation:

An element is a pure substance made up of only one type of atom. An atom is the smallest particle of an element that still holds the properties of that element.

Elements do not break down during normal laboratory activities, such as when they are heated or exposed to electric currents. They also do not break down when they undergo chemical reactions, such as reactions with acids.

answer; //the above statement is (false);

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.