A. Yes, there are no y-values that have more than one x-value B. No; there are y-values that have more than one x-value.
C. Yes; the graph passes the vertical line test.
D. No; the graph fails the vertical line test.

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?