Chapter 4 - An Introduction to Functions - 4-6 Formalizing Relations and Functions - Practice and Problem-Solving Exercises - Page 273: 35

Horizontal line: Yes Vertical line: No

Just consider this question in terms of the vertical line test. A horizontal line will always pass the vertical line test as there is no vertical line that can intersect a horizontal line twice, thus making horizontal lines functions. However, for a vertical line, another vertical line can be placed on top of it. This indicates that there are multiple outputs for one input, so this is not a function.