如图，四边形ABCD内接于⊙O，若∠AOC = 100°，则∠ABC的大小为（\quad）---Based on the image provided, only a geometric diagram is present. There is no text for the question, options, or any other related information. **Diagram Description:** * **Type:** Geometric figure. * **Main Elements:** * A circle is shown. * Point O is labeled, representing the center of the circle. * Four distinct points A, B, C, and D are located on the circumference of the circle. * Line segments connect the points in sequence: AB, BC, CD, and DA, forming an inscribed quadrilateral ABCD. * A diagonal of the quadrilateral, AC, is drawn. * Line segments connecting the center O to point A (OA) and to point C (OC) are drawn, which are radii of the circle. * **Labels:** The points are labeled as A, B, C, D, and O. * **Relationships:** Points A, B, C, and D are concyclic (lie on the same circle). O is the center of the circle. OA and OC are radii. ABCD is a cyclic quadrilateral. AC is a chord and a diagonal of ABCD.