How can I make 'replace-string' to access the kill-ring so that I have a choice of whatever is in the kill-ring please?

Currently I have this in my config -

#+BEGIN_SRC emacs-lisp

(global-set-key (kbd "M-s-x") 'replace-string)

#+END_SRC

- but I can't find any way of accessing the kill-ring so that I can choose what text, or whatever, to search and replace for.

Its probably something quite simple but its got me stumped at the moment, and I can't find a solution on the web using google.

Thanks
Sharon.
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