Area and volume SAT question

The correct answer is $31$. The circumference of a circle is equal to $2\pi r$ centimeters, where $r$ represents the radius, in centimeters, of the circle, and the diameter of the circle is equal to $2r$ centimeters. It's given that a circle has a circumference of $31\pi$ centimeters. Therefore, $31\pi =2\pi r$. Dividing both sides of this equation by $\pi$ yields $31=2r$. Since the diameter of the circle is equal to $2r$ centimeters, it follows that the diameter, in centimeters, of the circle is $31$.