请看解释 http://zhidao.baidu.com/question/323746250.html?device=mobile&ssid=0&from=1086k&uid=0&pu=usm@0,sz@1320_1002,ta@iphone_2_4.4_2_5.7&bd_page_type=1&baiduid=942BE5B3BD67F52987D2D831DF876A04&tj=www_normal_1_0_10_title