怎么清空表单中指定元素的值？

<script>
function control()
{   
    var sel=document.getElementsByName("opt")[0].value;
    if(sel=='1')
     {
          document.form.a.value="";
     }
    if(sel=='2')
     {
          document.form.b.value="";
     }
    if(sel=='3')
     {
          document.form.c.value="";
     }
}  
</script>

<form id="form" name="form" method="post" action="">
        文本框a：
        <input type="text" name="a" id="a" />
        复选框b：
        <input type="checkbox" name="b" id="b" />
        <select name="c" id="c">
                <option>下拉菜单c</option>
                <option value="1">001</option>
                <option value="2">002</option>
                <option value="3">003</option>
        </select>
        <select name="opt" id="opt" onChange="control();return false;">
                <option>选择</option>
                <option value="1">清空文本框a</option>
                <option value="2">清空复选框b</option>
                <option value="3">清空下拉菜单c</option>
        </select>
</form>