What is a one-to-one function?

a. A function f is called a one-to-one function if it has negative slope.
b. A function f is called a one-to-one function if it is a vertical line.
c. A function f is called a one-to-one function if it never takes on the same value twice.
d. A function f is called a one-to-one function if it is a horizontal line.
e. A function f is called a one-to-one function if it periodically takes on the same value.