A circle has center (2, 3). A line is tangent to the circle at the point (2, 8). What is the slope of the tangent line?

The radius from the center $(2, 3)$ to the point of tangency $(2, 8)$ is a vertical segment, since both points have the same $x$-coordinate. A tangent line is perpendicular to the radius at the point of tangency, so the tangent is horizontal: the line $y = 8$. A horizontal line has slope 0, choice A. Choice B (undefined) is the slope of the radius itself, which is vertical, so this confuses the radius and the tangent. Choices C (1) and D ($-1$) are slopes of diagonal lines, which do not follow from a vertical radius.