Should the plural of CSR or other abbreviations have an appostrophe?

Question:For example: CSRs/CSR's, RNs/RN's, or As/A's and Bs/B's on a report card, etc.

Answers:
About 20 years ago we were taught that it was correct to use apostrophes for plurals of capitals (A's and B's) and numbers (80's and 90's), but the convention has changed and so now we don't do it anymore. But we still do it for lowercase letters (mind your p's and q's).

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