Explanation:
when you have 2 points, there is always one line going through these 2 points.
in other words (which is an often used statement) : 2 points always make a line.
just imagine you are standing on one point looking to the other point. your friend is standing on the other point. you throw a line over there. when you both pull it taught, that creates a straight line.
how many more different straight lines (not arcs) can you make from you to your friend ? 0
the one straight line is THE straight line. if there is any other line, it would have to make a detour and hence would not be a straight line.