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.