For Juanita to get to the zoo she must take either the number 12 bus or else the subway. Everyone knows that the number 12 bus is not running this week; so although Juanita generally avoids using the subway, she must have used it today, since she was seen at the zoo this afternoon. The method of the argument is to ['show that something is the case by ruling out the only alternative', 'substitute a claim about what invariably occurs for a claim about what typically occurs', 'demonstrate that certain possibilities are not exclusive', 'assert that if something is true, it will be known to be true']