Where is the circumcenter of any given triangle?
a. the point of concurrency of the altitudes of the triangle
b. the point of concurrency of the perpendicular bisectors of the sides of the triangle
c. the point of concurrency of the bisectors of the angles of the triangle
d. the point of concurrency of the medians of the triangle

Statements reasons 1. ab ? cd; ad ? bc 1. given 2. ac ? ac 2. reflexive property 3. ? adc ? ? cba 3. ? 4. ? dac ? ? bca; ? acd ? ? cab 4. cpctc 5. ? dac and ? bca are alt. int. ? s; ? acd and ? cab are alt. int. ? s 5. definition of alternate interior angles 6. ab ? cd; ad ? bc 6. converse of the alternate interior angles theorem 7. abcd is a parallelogram 7. definition of parallelogram what is the missing reason in step 3?