The relative hardness of five minerals N, o, R, S, and T is,to be determined. One mineral is harder than another if drawing an edge of the first mineral across a surface of the second mineral produces a scratch; otherwise, the first mineral is either equally hard or, not  as hard as the second. The following results have so far been obtained.   N scratches O R scratches S O does not scratch T.  If O scratches S, which of the following must be true?