返回嵌套元素的索引(Return index of nested element)

 我有两个嵌套的ul包含链接。 
 
 我想在其父级ul找到被点击的索引 
 
<div id="main_nav">
<ul>
    <li>
        <ul>
            <li><a href="/url/">LINK</link></li>
            <li><a href="/url/">LINK</link></li>
            <li><a href="/url/">LINK</link></li>
        </ul>
    </li>
</ul>
<ul>
    <li>
        <ul>
            <li><a href="/url/">LINK</link></li>
            <li><a href="/url/">LINK</link></li>
            <li><a href="/url/">LINK</link></li>
        </ul>
    </li>
</ul>

</div>
 
 我正在使用以下代码： 
 
$("#main_nav ul ul a").click(function () {

    var index = $('#main_nav ul ul a').index(this);

    alert(index);
});
 
 这似乎返回parent of the parent而不是parent of the parent的a的索引（我希望这是有意义的）即，单击第二次ul中的第二个链接将返回索引4而不是1。 
 
 有人可以解释我哪里出错吗？ 我怀疑它很简单，但我无法弄明白。 
 
 非常感谢。 

I have two nested ul which contain links.
 
I want to find the index of the clicked a within it's parent ul
 
<div id="main_nav">
<ul>
    <li>
        <ul>
            <li><a href="/url/">LINK</link></li>
            <li><a href="/url/">LINK</link></li>
            <li><a href="/url/">LINK</link></li>
        </ul>
    </li>
</ul>
<ul>
    <li>
        <ul>
            <li><a href="/url/">LINK</link></li>
            <li><a href="/url/">LINK</link></li>
            <li><a href="/url/">LINK</link></li>
        </ul>
    </li>
</ul>

</div>
 
I'm using the following code:
 
$("#main_nav ul ul a").click(function () {

    var index = $('#main_nav ul ul a').index(this);

    alert(index);
});
 
which seems to return the index of the a within the parent of the parent instead of just the parent (I hope that makes sense) i.e. Clicking the second link in the second ul returns an index of 4 instead of 1.
 
Could somebody explain where I'm going wrong with this? I suspect it's something simple but I can't figure it out.
 
Many thanks.