Answer:
A
Explanation:
the exterior angle of a triangle is equal to the sum of the 2 opposite interior angles.
140° is an exterior angle of the triangle , then
∠ B + 90° = 140° ( subtract 90° from both sides )
∠ B = 50°
∠ ACB and ∠ DCB are a linear pair and sum to 180° , that is
∠ ACB + 140° = 180° ( subtract 140° from both sides )
∠ ACB = 40°