Mateo is constructing an equilateral triangle inscribed in a circle with center P. He draws the diameter of the circle through center P using a straight edge. What is his next step?

A. Use a straight edge and compass to find lines perpendicular to the endpoints of the diameter.
B. Open his compass to a width equivalent to the radius of the circle.
C. Open his compass to a width equivalent to the diameter of the circle.
D. Use a straight edge and compass to find the perpendicular bisector of the diameter.