From Pythagorean theorem, we know that OB= root number 3, then OA, OB and AB are divided by root number 3 at the same time, and we know that triangle OAB is a right triangle with an angle of 30 degrees. So the angle OBC is equal to 60 degrees, so the angle BOC is equal to 120 degrees, and the lower arc BC is equal to π/3 * OB = (root number 3÷3)*π, which is a. 。