For Megan to get to the grocery store, she must take either the bus or the subway. The bus is not running this week, so although Megan generally does not like to take the subway, she must have used it today, since she was seen at the grocery store today. Which one of the following most accurately describes the method of reasoning used to reach the conclusion above? ['showing that there are additional alternatives that are unknown', 'showing that certain possibilities are not exclusive', 'showing that no exceptions could apply', 'showing that something must have occurred by excluding the only alternative']