<a class="s">1</a> <a class="s">2</a> <a class="s">3</a> <a class="s">4</a> <a class="s">5</a> <a class="s">6</a> <a class="s">7</a>
获取相同类名的值，从上到下的顺序，从0开始
<script>
alert($(".s").eq(5).html()); </script>
