各位大侠，急急。。。JSP和Tomcat的问题

我做了一个a.jsp页面，上面显示一行test JSP！然后做了一个HelloServlet.java的Servlet
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class HelloServlet extends HttpServlet {
  public void doGet(HttpServletRequest request,
  HttpServletResponse response)
  throws ServletException, IOException
  {
PrintWriter out = response.getWriter();
out.println("Hello World");
  }
}
web.xml配置如下：
<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE web-app
    PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
    "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>
<servlet>
<servlet-name>HelloWorld </servlet-name>
<servlet-class>HelloServlet </servlet-class>
</servlet>

<servlet-mapping>
<servlet-name>HelloWorld </servlet-name>
<url-pattern>/Hello </url-pattern>
</servlet-mapping>
</web-app>

当我在IE中输入http://10.26.70.30:8080/FirstWA/a.jsp是能显示 “test JSP”
但是当我输入http://10.26.70.30:8080/FirstWA/Hello时就不能显示，报错如下：

HTTP Status 500 -

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