阅读下面代码片段: Servlet1: request.setAttribute("username", ""); String username=(String) request.getAttribute("username"); System.out.println("username:"+username); request.setAttribute("username", null); username=(String) request.getAttribute("username"); System.out.println("username:"+username); 当使用浏览器访问Servlet1时控制台会输出什么结果( ) A. B. null C. null D. null null