未定义的索引PHP代码

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            <div class="grid--cell mb0 mt8">Closed <span title="2013-04-10 16:20:13Z" class="relativetime">7 years ago</span>.</div>
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I have this php that uses ajax

cotizaciones.php

        <script>
        function showUser(str)
        {
            if (str == "")
            {
                document.getElementById("txtHint").innerHTML = "";
                return;
            }
            if (window.XMLHttpRequest)
            {// code for IE7+, Firefox, Chrome, Opera, Safari
                xmlhttp = new XMLHttpRequest();
            }
            else
            {// code for IE6, IE5
                xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
            }
            xmlhttp.onreadystatechange = function()
            {
                if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
                {
                    document.getElementById("txtHint").innerHTML = xmlhttp.responseText;
                }
            }
            xmlhttp.open("GET", "getuser.php?q=" + str, true);
            xmlhttp.send();
        }
    </script>

I have another script as above, except that it is called calcularCuotas(cuotas) and the last lines are:

xmlhttp.open("GET", "getuser.php?c=" + str, true);
                xmlhttp.send();

The body tag contains:

<select name="users" onchange="showUser(this.value)">
<select name="cuotas" onchange="calcularCuota(this.value)">

The file getuser.php

    <?php 
$q = $_GET["q"]; 
$c = $_GET["c"]; 
?>

When I try to use it, php throws this error:

Notice: Undefined index: c in C:\wamp\www\Cotizacion\getuser.php on line 3

Thanks for help :)

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