Let the set X:−{A,B,C} represent 3 buildings on campus. Suppose we wish our distance to be the time it takes to walk from one building to the other. It takes 5 minutes either way between buildings A and B. However, building C is on a hill and it takes 10 minutes from A and 15 minutes from B to get to C. On the other hand it takes 5 minutes to go from C to A and 7 minutes to go from C to B, as we are going downhill. Do these distances define a metric? If so, prove it, if not, say why not.