Select the three true statements.
1. pi is the area of a circle of radius 1.
2. pi is the area of a circle of diameter 1.
3. pi is the circumference of a circle of radius 1.
4. pi is the circumference of a circle of diameter 1.
5. pi is the constant of proportionality relating the diameter of a circle to its circumference.
6. pi is the constant of proportionality relating the radius of a circle to its area.