A Grindstone of Radius 4

Multiple Choice

A grindstone of radius 4.0 m is initially spinning with an angular speed of 8.0 rad/s. The angular speed is then increased to 12 rad/s over the next 4.0 seconds. Assume that the angular acceleration is constant.
-What is the average angular speed of the grindstone?

A) 0.5 rad/s
B) 2.0 rad/s
C) 4.0 rad/s
D) 8.0 rad/s
E) 10 rad/s