The slope of a horizontal line is always _____.

Question Analysis
This question tests the basic definition of the slope of a horizontal line.

Key Concept Explanation
A horizontal line has no change in the y-value as the x-value changes, resulting in a slope of 0.

Step-by-step Solution
1. Recall the definition of a horizontal line: it is a line where the y-coordinate remains constant for any change in the x-coordinate....