An interview question for HTML, which can be realized by jquery

Please read the following HTML, and answer the question.

<body style="text-align: center">
<script language="JavaScript">
    function prepareUpdateRow(){
        var frm = document.resultForm;
        if(selectedCheckboxCount(frm) < 1){
            alert("");
            return ;
        }else if(selectedCheckboxCount(frm) > 1){
            alert(", ");
            return ;
        }
        frm.action="training.training_P_TB_FeedbackPrepareUpdate.do";
        frm.submit();    
    }             
</script>

<table border="1">
    <tr>
        <td></td>
        <td></td>
    </tr>
    <tr>
        <td><input type="checkbox" name="cbx1" value="51" /> </td>
        <td>EOS </td>
    </tr>
    <tr>
        <td><input type="checkbox" name="cbx2" value="52" /> </td>
        <td>EOS </td>
    </tr>
    <tr>
        <td><input type="checkbox" name="cbx3" value="53" /> </td>
        <td>EOS </td>
    </tr>
    <tr>
        <td><input type="checkbox" name="cbx" value="54" /> </td>
        <td>1--515 </td>
    </tr>
</table>
<a href="javascript：prepareUpdateRow();"></a>
</body>

in the above code, the implementation of the function selectedCheckboxCount is missing. Please help the implementation (to determine how many checkbox are selected)
hint: the method to determine whether checkbox is selected is: object.checked = true/false;