Plural forms of abbreviations?
As I know, if we have an awkward plural (usually of a letter, a number, or an unusual abbreviation), we can use an apostrophe to assist our readers.
Hawaii is spelt with two i's.
She used six and's in one sentence.
However, in some specific cases, I don't know whether I should use "s" or " 's". For example, is MCs and MC's, CDs and CD's the same?