What is the slope of a vertical line?
Answer: D
A vertical line has a constant x-coordinate for all its points. Let two points be \((x, y_1)\) and \((x, y_2)\).
The slope formula is \(m = \frac{y_2-y_1}{x_2-x_1}\).
Here, the denominator \(x_2-x_1\) becomes \(x-x=0\).
Division by zero is undefined, so the slope of a vertical line is undefined.