给定一个Servlet的代码片段如下所示： public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException{ String str = "world"; HttpSession session = request.getSession(); session.setAttribute("str", str); ___________________ } 要取出session中的值，下划线处的代码可以是 A. String s = session.getParameter("str"); B. String s = (String)session.getAttribute("str"); C. Object o = session.getAttribute("str"); D. Object o = (Object)session.getParameter("str");