In a circle with center C and radius 6, minor arc AB has a length of 4pi. What is the measure, in radians, of central angle ACB? PLZ EXPLAIN A)2pi/9B) pi/3C)2pi/3D)4pi/3

To solve this problem, we need to know that 
arc length = r θ  where θ is the central angle in radians.

We're given
r = 6 (units)
length of minor arc AB = 4pi
so we need to calculate the central angle, θ
Rearrange equation at the beginning,
θ = (arc length) / r = 4pi / 6 = 2pi /3

Answer: the central angle is 2pi/3 radians, or (2pi/3)*(180/pi) degrees = 120 degrees