For my official Master's thesis, i need to know how to write the plural form of abbreviations. Let's say the abbreviation is CERN. And let's say we have two. Do we have two CERNs or two CERN's? My preference goes to the latter form, with apostrophe, but i'm not sure what the official ruling on this is. Sorry if this should be in an other forum, but i couldn't find a better one. Thanks in advance!