Answer:
D. 40
Explanation:
A central angle is equal to the measure of its corresponding arc.
angle AOB is a central angle and it's corresponding arc is arc AB
this means that arc AB = measure of angle AOB
If angle AOB = 80 degrees then arc AB equals 80 degrees too.
An inscribed angle is equal to half the measure of its intercepted arc.
Angle ACB is an inscribed angle and the arc it intercepts is arc AB
This means that angle ACB = 1/2 of arc AB
We have found that the measure of arc AB is 80 degrees.
This means that angle ACB = 1/2 of 80 which is 40