One thing that (excuse me) has been covered is the use of the apostrophe to indicate plural amounts of letters and numbers.
"Bs" or "B's."
"1980s" or "1980's"
Unfortunatly, this matter in a flux, and is really a question of style based on appearance and personal preference. There are conflicting reports on proper usage in this matter from several reliable sources. There is one rule that seems to be constant, however. If the letter (or group of letters) is an S or ends in S, it is proper to use the apostrophe to pluralize it, such as: "There are five S's in that word" and not "There are five Ss in that word".
It is also generally considered improper to use an apostrophe to pluralize abbreviations such as CD. This, however, also is conflicted.
The bottom line? Do it however you want to (or whichever way your teacher wants for assignments).