单选按扭的问题

---------------------index.html-----------------------------------
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
</head><body>
<form id="form1" name="form1" method="post" action="result.jsp">
  <label></label>
  <p>请输入你的选择：</p>
  <p>
    <label>
    <input name="selected" type="radio" value="石头" checked="checked" />
    石头</label>
    <label>
    <input name="selected" type="radio" value="剪刀" />
    剪刀</label>
    <label>
    <input type="radio" name="selected" value="布" />
    布</label>
    <label></label>
  </p>
  <p>
    <label>
    <input type="submit" name="Submit" value="提交" />
    </label>
  </p>
  <p>&nbsp;</p>
</form>
</body></html>
--------------------------------------------------------------------
---------------------------result.jsp-----------------------------------
<%@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" errorPage="" %>
<html ><body bgcolor="white"><hr>
你的选项： <br>
<% 
String selected_t= request.getParameter("selected");
out.println(selected_t);
%></font>
</body>
</html>
-------------------------------------------------每次运行,结果     "你的选项:null"求救