13. In reference to a circle the value of \( \pi \) is equal to
(a) \( \tfrac{\text{area}}{\text{circumference}} \)
(b) \( \tfrac{\text{area}}{\text{diameter}} \)
(c) \( \tfrac{\text{circumference}}{\text{diameter}} \)
(d) \( \tfrac{\text{circumference}}{\text{radius}} \)
By definition, \( \pi = \tfrac{\text{circumference}}{\text{diameter}} \).