Which circles have a circumference of 20pi units? choose all answers that are correct. a) a circle with a diameter of 40 units; b) a circle with a radius of 10 units; c) a circle with a diameter of 20 units; d) a circle with a radius of 20 units?

So, keep in mind that for a circle, circumference equals pi times the diameter.  So, C=pi*d.  In this problem, 20pi=pi*d.  We can solve for d by dividing both sides by pi, for inverse operations: 20pi/pi=(pi*d)/pi.  d=20.  So, the diameter is 20 units.  Finally, keep in mind that the radius is half the diameter.  So, the radius is 20/2 units, or 10 units.

B, C