若集合A＝{ a，{a}，{1，2}}，则下列表述正确的是（　）．

{<em>a</em>，{<em>a</em>}}<img style、"vertical-align: middle;" src="http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1207011/2/48763/003.gif" alt="" width="13" height="13"><em>A</em>

{<em>a</em>}<img style、"vertical-align: middle;" src="http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1207011/2/48765/004.gif" alt="" width="16" height="16"><em>A</em>

{1，2}<img style、"vertical-align: middle;" src="http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1207011/2/48764/image002.png" alt="" width="13" height="16"><em>A</em>

<em><img style、"vertical-align: middle;" src="http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1207011/2/48766/006%20%281%29.gif" alt="" width="17" height="19"><img src="http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1207011/2/48766/003.gif" alt="" width="13" height="13">A</em>