A. True. If it is possible to pass a single horizontal line through more than one point on the function curve, then the function is not one-to-one (it fails the horizontal line test).
B. True. Any given input must lead to exactly one output.
C. False. The vertical line test is used to determine if a relation is a function or not.
D. False. The domain of a sequence is the set of natural numbers {1, 2, 3, ...}
