According to the meaning of the question, AC is parallel to the X axis and OA is parallel to BC. (This question examines the properties of parallel lines in combination with the coordinate system. It seems simple, but if you don't pay attention, you will miss the solution. ) Two straight lines are parallel, with complementary internal angles and equal internal dislocation angles. Therefore, it can be concluded that the angle ACB is 60 or120, and only the angle is given, but the positive and negative directions are not specified. That is, the line segment OA may be in the first, second, third and fourth quadrants. Among them, the third and fourth quadrants are symmetrical with the second and first quadrants about X respectively ... only two are drawn. Hope pure hand to adopt, thank you!