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 each value of the input variable is paired with exactly one value of the output variable. c. the vertical line test may be used to determine whether a function is one-to-one. d. a sequence is a function whose domain is the set of real numbers.