Select all of the true statements. Explain only why the statements are true.
A. pi is the area of a circle of radius 1.
B. pi is the area of a circle of diameter 1.
C. pi is the circumference of a circle of radius 1.
D. pi is the circumference of a circle of diameter 1.
E. pi is the constant of proportionality relating the diameter of a circle to its circumference.
F. pi is the constant of proportionality relating the radius of a circle to its area.