According to Postulate 5, otherwise known as the Plane Line Postulate, if two points lie in a plane, the line containing those points also lies in the same plane. In other words, if you have a square plane, for example, and you put two points within the plane and you draw a line that connects both of those points. The line you created using those two points, or the line that connects the points, are within the same plane.
Your answer is C.
