Which expression gives the circumference of a circle in terms of its radius r?

• A. C = π d
• B. C = π r
• C. C = 2π r
• D. C = r^2

Answer: (C)

The circumference relates directly to the radius with a factor of 2π. Starting from the common form that uses diameter, C = πd, and knowing the diameter is twice the radius (d = 2r), substitute to get C = π(2r) = 2πr. That is the expression that correctly expresses circumference in terms of the radius.

The other forms don’t fit for this purpose: using πd requires converting to radius first, which leads to 2πr anyway; a form with πr would miss the factor of 2, giving half the actual circumference; and r^2 is the area formula, not related to circumference.