Answer:
A) this has a problem because does not explicit the dimension.
For example, the set {1, 3}
Is the set of all points that are at the same distance (1 unit) from the point x = 2.
If we write the definition as:
"A plane figure whose points are at a fixed distance to some given point, the center".
Then we are defining that is a "plane figure" so we need a plane to have a figure, so we need to have two dimensions, then there is no mistake in this definition.
B) The undefined terms are the terms without a definition, those are point, line, plane and set.
To have a circle we need a plane (because the circle "lives" in a plane) And the circle itself is the set of all the points that are equidistant to a point that is in that plane.
So we are using at least 3 of the undefined terms to define a circle.
