如果select列表框设成多选，在javascript里应该怎么得到他被选中的值？

<select name= "test " onclick= "javascript:Select() ">
<option value= "a "> a </option>
<option value= "b "> b </option>
<option value= "c "> c </option>
<option value= "d "> d </option>
</select>

在javascript里面
function Select(){
for(var i=0;i <document.all.test.length;i++)
if(document.all.test[i].checked==true
alert(document.all.test[i])
}
这样就可以拉

<html> 
<head> 
<title>   New   Document   </title> 
<script> 
function   showSelectedValue() 
{ 

var   selectedValue   =   " "; 
var   objSelect   =   document.form1.test; 
for(var   i   =   0;   i   <   objSelect.options.length;   i++) 
{ 
if   (objSelect.options[i].selected   ==   true) 
selectedValue   +=   objSelect.options[i].value   +   "\r\n "; 
} 
alert(selectedValue); 
} 
</script> 
</head> 
<body> 
<form   name= "form1 "> 
<select   name= "test "   multiple   > 
<option   value= "a "> a </option> 
<option   value= "b "> b </option> 
<option   value= "c "> c </option> 
<option   value= "d "> d </option> 
</select> 
<input   type= "button "   value= "showSelectedValue "   onClick= "javascript:   {showSelectedValue();} "> 
</form> 
</body> 
</html>