I created a menu with Radio Buttons in XPCE prolog, but my radio buttons are showing on the same line, and I want to have each one on a separate line.

My code is the following:

new(D, dialog('title')), send(D, size, size(500,500)), send(D, append, new(Op, menu(options, marked))), send(Op, append, option1), send(Op, append, option2), send(Op, append, option3), send(Op, size, size(300,300)), send(D, display, Op, point(100, 40)), send(D, append(new(B1,button(ok, message(D, return, Op?selection))))), send(D, display, B1, point(100, 100)), send(D, append(button(cancel, message(D, return, @nil)))), send(D, default_button(ok)), get(D, confirm, Rval), free(D).

Thanks in advance