The Equation circle centered at (m, n) wibth radius R is :

Since comparative data circuit is :

After all, this is a circle centered at the point (c, c-) with radius | c |.

I think that this trick question ... To know that the radius must always be positive that this is a definition (c tells us that real, not a given that the charges). For c = 0 of course it is a circle centered at the origin and radius 0, ie a point so it meets the conditions are empty. Any c different from 0 (positive or negative) point (c, c-) of course exists, then there is only that | c | then there is always such a positive cycle