Actually it is not entirely true as posted above.
Earlier, 's was used for pluralization and was (not sure about now) considered formal English.
FYI: If you see cases in GMAT/GMAT Prep with 's used as plural then mostly it will not test the plural singular form of that noun [all options will be using it in the same way]See the link: GMAT Guru's Comment [Bob]http://www.urch.com/forums/gmat-sentenc ... post609784
as you can see the above link has GMAT Prep question and is using CEO's for plural CEOs.
Consider kudos for the good post ...
My debrief : journey-670-to-720-q50-v36-long-85083.html