Question 55 page 828 HFSJ first edition

[Mamadou Touré]

This snippet code is from HFSJ 1st edition.
I don't really understand why the response is B:

 Given:
11. public class ServletX extends HttpServlet {
12. public void doGet(HttpServletRequest req,
13. HttpServletResponse res)
14. throws IOException, ServletException {
15. req.getSession().setAttribute(�key�, new X());
16. req.getSession().setAttribute(�key�, new X());
17. req.getSession().setAttribute(�key�, �x�);
18. req.getSession().removeAttribute(�key�);
19. }
20. }
and given a listener class:
11. public class X implements HttpSessionBindingListener {
12. public void valueBound(HttpSessionBindingEvent event) {
13. System.out.print(�B�);
14. }
15. public void valueUnbound(HttpSessionBindingEvent event) {
16. System.out.print(�UB�);
17. }
18. }
Which logging output would be generated by an invocation of the doGet method?
A. UBUBUB
B. BBUBUB
C. BBUBUBB
D. BUBBUBB
E. BBUBUBBUB
F. BBUBBUBBUB
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