disabled เมื่อยังกรอก ไม่ครบ

[raccoon]

<script>
function check() {
obj = document.form1
if(obj.a.value != "" && obj.b.value != "") {
obj.Submit.disabled = false
}
}
</script>
<form id="form1" name="form1" method="post" action="">
<p>
<select name="a" id="a" onchange="check();">
<option value="">เลือก</option>
<option value="1">test</option>
</select>
</p>
<p>
<input name="b" type="text" id="b" onkeyup="check();" />
</p>
<p>
<input type="submit" name="Submit" value="Submit" disabled="disabled" />
</p>
</form>