Answer:
D. 100°
Explanation:
ABCD is a cyclic quadrilateral because it is inscribed in a circle.
The opposite angles of a cyclic quadrilateral sum to 180°. So, to know the measure of angle B, we will use the following equation:
m∠B + m∠D = 180
Then, solving for m∠B, we get:
m∠B + 80 = 180
m∠B + 80 - 80 = 180 - 80
m∠B = 100
Therefore, m∠B is 100°
