Question

If you had to put up a fence around a circular baseball field, you would need to know the _________?

Answer 1

Circumference is  Perimeter except for circles.

Perimeter is the total border of a shape.

Knowing this...

The answer is 
If you had to put up a fence around a circular baseball field, you would need to know the "Circumference"?

Answer 2

You would need to know the diameter of the field, and from that, you need to get the circumference of the field by using the formula C = πd.

By getting the circumference, you can therefore determine the perimeter/length of the fence you will need to enclose it.