select下拉框，设置不被选择方式
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title> New Document </title>
<meta name="Generator" content="EditPlus">
<meta name="Author" content="">
<meta name="Keywords" content="">
<meta name="Description" content="">
</head>
<body>
<select>
<option value="">1</option>
<option value="">2</option>
<optgroup label="3"></optgroup>
<optgroup label="4"></optgroup>
<optgroup label="5"></optgroup>
</select>
</body>
</html>
