IE7下z-index失效问题

看代码:

HTML

                <div class="select-wrap">
                    <div class="select-name">院系</div>
                    <div class="arrow"><img src="./img/arrow.png"></div>
                    <span class="selected-text"></span>
                    <div class="select">
                        <ul>
                            <li><a href="javascript:void(0);">院系1</a></li>
                            <li><a href="javascript:void(0);">院系2</a></li>
                            <li><a href="javascript:void(0);">院系3</a></li>
                        </ul>
                    </div>
                </div>
                <div class="select-wrap">
                    <div class="select-name">类型</div>
                    <div class="arrow"><img src="./img/arrow.png"></div>
                    <span class="selected-text"></span>
                    <div class="select">
                        <ul>
                            <li><a href="javascript:void(0);">类型1</a></li>
                            <li><a href="javascript:void(0);">类型2</a></li>
                            <li><a href="javascript:void(0);">类型3</a></li>
                        </ul>
                    </div>
                </div>

CSS

.select-wrap{
    position: relative;
    width:288px;
    height:34px;
    margin-top: 15px;
    border: 1px solid #cdcbcb;
}
.select-name{
    float:left;
    width:48px;
    height:34px;
    line-height: 34px;
    text-align: center;
    color:#7b7979;
    background-color: #e6e6e6;
}
.selected-text{
    display: inline-block;
    float:left;
    width:190px;
    height:34px;
    font-size: 14px;
    padding-left: 10px;
    line-height: 34px;
}
.arrow{
    float:left;
    width:32px;
    height:16px;
    /*border: 1px solid red;*/
    margin-top: 10px;
    text-align: center;
    border-right: 1px solid #a8a8a8;
}
.arrow img{
    cursor: pointer;
    vertical-align: middle;
    margin-top: -8px;
    *margin-top:-3px;
}
.select{
    position:absolute;
    z-index:;
    top:34px;
    right:-1px;
    width:210px;
    background-color: #fff;
}
.select ul{
    border: 1px solid #a8a8a8;
    border-top:;

}
.select li:hover{
    background-color:#a8a8a8 ;
}
.select li a{
    display: block;
    height:20px;
    line-height: 20px;
    padding-left: 10px;
    font-size: 14px;
}

正常情况下,Chrome,FF:

IE7及360兼容模式下:

原因:ie中子元素的优先级要取决于其父元素的优先级,需要设置父元素的z-index

解决方法:因为有两个同一class的div,绝对定位的ul的父元素就是.select-wrap,但是如果给两个.select-wrap设置一样的z-index值,还是没有效果,因为ul要遮挡的下面的元素仍然是.select-wrap,也就是说ul的父元素的优先级和下面要遮挡的元素的优先级相同,仍然无效,解决方法就是给两个select-wrap设置不同的z-index值即可.