P7-c Check your style guide to determine whether to include an apostrophe to form the plural of a number, a letter, or an abbreviation.
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Some style guides encourage writers to form the plural of a number, a letter, or an abbreviation by adding -’s.
However, the MLA Handbook (the most commonly used style guide in courses in English literature and composition) does not. So check with your instructor to be sure what is appropriate.