Which of the following statements are true regarding functions? Check all that apply.

a.) the horizontal line test may be used to determine whether a function is one-to-one.

b.) a function is a relation in which multiple values of the input variable are paired with at least one of the output variable.

c.) a sequence is a function whose domain is the set of rational numbers.

d.) the vertical line test may be used to determine whether a relation is a function.