Which of the following statements are true regarding functions? check all that apply.
a. a function is a relation in which multiple values of the input variable are paired with at least one of the output variable.
b. a sequence is a function whose domain is the set of rational numbers.
c. the horizontal line test may be used to determine whether a function is one-to-one.
d. the vertical line test may be used to determine whether a relation is a function.