阅读下面代码片段:Servlet1: request.setAttribute("username", "张三"); 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. 张三 张三
B. 张三 null
C. null 张三
D. null null