Arc CD is 2 thirds of the circumference of a circle. What is the radian measure of the central angle?

A full circle measures 2π radians. Since arc CD is two-thirds of the circumference, its central angle is two-thirds of 2π: (2/3)(2π) = 4π/3 radians. Therefore, the correct choice is 4π/3 radians.

AI-written and checked against the verified answer.