Arc CD is [tex]\frac{2}{3}[/tex] of the circumference of a circle. What is the radian measure of the central angle?

a.) [tex]\frac{2\pi}{3}[/tex]
b.) [tex]\frac{3\pi}{4[/tex]
c.) [tex]\frac{4\pi }{3}[/tex]
d.) [tex]\frac{3\pi}{2}[/tex]