This problem will check your understanding of the concept of functions. Let me remind you the definition of a function:

Definition of a Function: A function is a rule or correspondence which associates to each number x in a set A a unique number f(x) in a set B.

Which of the followings can not be a function?

A. T(t)is the temperature of hot water after time t.
B. C(w) is the set of mailing a first-class letter with weight w.
C. P(t) is the human population of the world at time t
D. N(x) is a person whose last name is John
E. All of the above
F. None of the above