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为什么我的li属性的导航文本一开始就不是在顶部，所以在设置line-height=40px时，文本就没有在导航条中居中了，而是跑到导航条下面了呢？

.nav{height:40px;}

.nav_left{width:10px;

background:url(file:///C|/Users/Administrator/Documents/%E6%9C%AA%E5%91%BD%E5%90%8D%E7%AB%99%E7%82%B9%202/nav_left.jpg)no-repeat;}

.nav_mid{width:980px;background:url(file:///C|/Users/Administrator/Documents/%E6%9C%AA%E5%91%BD%E5%90%8D%E7%AB%99%E7%82%B9%202/nav_bg.jpg)repeat-x;}

.nav_right{width:10px;background:url(file:///C|/Users/Administrator/Documents/%E6%9C%AA%E5%91%BD%E5%90%8D%E7%AB%99%E7%82%B9%202/nav_right.jpg)no-repeat;height:40px;float:left;}

.nav_left,.nav_mid{height:40px;float:left;}

.nav_mid_left,.nav_mid_right{float:left;}

.nav_mid_left{width:680px}

.nav_mid_right{width:300px}

.nav_mid_left li{float:left;list-style-type:none;width:100px;text-align:center;}