We are given that angle AOB is a central angle of circle O and that angle ACB is a circumscribed angle of circle O. We see that AO ≅ BO because . We also know that AC ≅ BC since . Using the reflexive property, we see that . Therefore, we conclude that △ACO is congruent to △BCO by the .
Selection 1:all radii of the same circle are congruent
Selection 2:tangents to a circle that intersect are congruent
Selection 3:side CO is congruent to side CO
Selection 4:SSS congruency theorem