A wall measures $$\rm 8m$$ by $$\rm 20m$$.  Pip wants to wallpaper this wall with square pictures.  What is the length of the largest picture he can use so that all of the wall will be covered? 
选项为:A:$$\rm 2m$$  B:$$\rm 3m$$  C:$$\rm 4m$$  D:$$\rm 5m$$  请回答其中一个选项.