Pi is defined as the ratio of the circumference of a circle to the diameter of that circle. which of the following correctly explains why the formula for the circumference of a circle is 2 pi r? a. two times pi equals the distance from one side of the circle to the other. when you multiply around the circle, or the circumference. b. pi times r equals the diameter of the circle. the diameter is a half circle, so when you multiply it by 2, you get the distance around the entire circle, or the circumference. c. two times r equals the diameter of the circle. pi is needed for all circle formulas, so you multiply by pi since you are finding the circumference. d. two times r equals the diameter of the circle. pi equals the circumference divided by the diameter. when you multiply, the diameter is in both the numerator and the denominator, which cancels out, leaving the circumference.