Since segment AB is a diameter of the circle, it follows that arc ADB is a semicircle. Thus, the circumference of the circle is twice the length of arc ADB which is 2(8Ï€) = 16Ï€. Since the circumference of a circle is 2Ï€ times the radius of the circle, the radius of this circle is 16Ï€ divided by 2Ï€, which is equal to 8. Choices A, B, and D are incorrect and may be the result of losing track of factors of 2 or of solving for the diameter of the circle instead of the radius. For example, choice D is the diameter of the circle.