premise: A store sells shirts only in small, medium, and large sizes, and only in red, yellow, and blue colors. Casey buys exactly three shirts from the store. A shirt type consists of both a size and a color. Casey does not buy two shirts of the same type. Casey does not buy both a small shirt and a large shirt. No small red shirts are available. No large blue shirts are available. If Casey does not buy a medium yellow shirt, which one of the following must be true?
hypothesis: Casey buys either a medium red shirt or a medium blue shirt.