All human beings are either optimist or pessimists. Joey is not a pessimist. Therefore, Joey is an optimist. Which one of the following is assumed by the argument above?

A. Joey is a human being.

B. Optimists are never pessimists about anything.

C. Joey does not believe himself to be a pessimist

D. Joey does not allow disappointments to upset him

E. More People are optimists than are pessimists.