费的精力增多 ，因此，不满意度升高．当住第n层楼时，上下楼造成的不满意度为n．但高处空气新鲜，环境较安静，因此，随着楼层的升高，环境不满意度降低．设住第ｎ层楼时不满意度为，则此人应选（   ）                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          

A．１楼　　　 B．２楼　         C．３楼　     D．４楼