The circumference is a circle is represented by the equation c=pi•r^2 or pi•d. If you know d=9 and c=28.26 then just substitute the values into the second formula (c=pi•d) so you’ll have 28.26=pi•9. Dividing 9 from both sides isolates pi so you’ll have 28.26/9=pi. I believe C is the right answer.
