如何选取textarea的特定行

<SCRIPT language="javascript">
function getTextRange(num, oTextarea)
{
    var range = oTextarea.createTextRange();
    var rect = range.getClientRects();
    var left = rect[0].left;
    if(num > rect.length - 1 || num < 0)
        return;
    if(num == 0)
    {
       var right = rect[0].right;       range.moveEnd("character",-range.text.length);   
       while(range.offsetLeft + range.boundingWidth < right)
       {
           range.expand("character");
       }
    }
    else
    {
        var right = rect[num].right;        var range = getTextRange(num - 1, oTextarea);
        range.moveStart("character",range.text.length + 1);
        while(range.offsetLeft + range.boundingWidth < right)
        {
            range.expand("character");
        }
        if(range.offsetLeft > left)
        {
            range.moveStart("character",-1);
        }
    }
    range.moveEnd("character",+1);
    if(range.text.indexOf("\r")>=0)
    {
        range.moveEnd("character", range.text.indexOf("\r")-range.text.length+1);
    }else{
        range.moveEnd("character",-1);
    }
    return range;
}function getText(num)
{
    var range = getTextRange(num,Textarea1)
    if(range != null)
    {
        range.select();
    }
}
</SCRIPT><textarea cols="50" rows="10" id="Textarea1">
fsdasdasi
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaafsdasdasiaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
vsdasdasu
fsdasdasiaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bsdasdasd
</textarea><br><br><input type="button" onClick="getText(0)" value="选第1行">
<input type="button" onClick="getText(1)" value="选第2行">
<input type="button" onClick="getText(2)" value="选第3行">
<input type="button" onClick="getText(3)" value="选第4行">
<br>
<input type="button" onClick="getText(4)" value="选第5行">
<input type="button" onClick="getText(5)" value="选第6行">
<input type="button" onClick="getText(6)" value="选第7行">
<input type="button" onClick="getText(7)" value="选第8行">