The answers depend on what you want to "do" and what information you have.
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Multiply each of the diameters by pi (pi = 3.14159265 or 3.14 for rough approximation) to find the circumferences of the circles.
You need only know the radius of each circle to determine that they are congruent. If the radii are identical, the circles are identical. This can also be determined by comparing the diameters (twice the radii), or the circumferences, or the areas of the circles. In all cases, if the parameters are identical, the circles are identical.
They are its circumferences
There are infinite amount of diameters.
Only one.