Which simple graphical procedure determines whether a curve $y=g\left(x\right)$ represents a function of $x$?

• A. Use the horizontal line test: if every horizontal line intersects at least once, it's a function
• B. Check whether the graph passes through the origin; it is a function only if it does
• C. Use the vertical line test: if every vertical line intersects the graph at most once, it's a function
• D. Count the number of peaks and troughs; only functions with fewer than two peaks are included

Answer: (C)