Roberta is irritable only when she is tired, and loses things only when she is tired. Since she has been yawning all day, and has just lost her keys, she is almost certainly irritable. The reasoning above is flawed in that it ["takes a necessary condition for Roberta's losing things to be a sufficient condition", 'generalizes on the basis of a single instance', 'assumes the conclusion that it sets out to prove', "takes a necessary condition for Roberta's being irritable to be a sufficient condition"]