若集合A＝{2，a，{ a }，4}，则下列表述正确的是(    )．

{<em> a</em> }<img style、"vertical-align: middle;" src="http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1206997/4/48774/004.gif" alt="" width="16" height="16"><em>A</em>

{<em>a</em>，{<em> a</em> }}<img style、"vertical-align: middle;" src="http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1206997/4/48771/003.gif" alt="" width="13" height="13"><em>A</em>

{2}<img src、"http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1206997/4/48773/003.gif" alt="" width="13" height="13"><em>A</em>

<em><img style、"vertical-align: middle;" src="http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1206997/4/48772/006%20%281%29.gif" alt="" width="17" height="19"><img src="http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1206997/4/48772/003.gif" alt="" width="13" height="13">A</em>