A is incorrect because that means that all squigly lines that have ends that meet are circles, which is false. B is incorrect because this is not very precise as a definition. Also, compasses are not completely perfect, so the points are slightly off from being completely equidistant from the point. C is incorrect because that's describing a sphere (notice how it says space instead of plane).
When rounding the 4 you look to the right of the number, which is the 4, and if the number is higher than 5 you would change the 4 to 5. Therefore: 71.5
