设集合A={a}，则A的幂集为(    )．

、{{<em>a</em>}}

、{<em>a</em>，{<em>a</em>}}

{<img style、"vertical-align: middle;" src="http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1207026/5/48781/006%20%281%29.gif" alt="" width="17" height="19">，{<em>a</em>}}

{<img style、"vertical-align: middle;" src="http://hebei.ouchn.cn/pluginfile.php/19780/question/answer/1207026/5/48782/006%20%281%29.gif" alt="" width="17" height="19">，<em>a</em>}