There's a sentence in my textbook:
My sister got A's in her final exam.
Shouldn't be As instead in this case?
Can 's also be a kind of stylish for the plural?
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This is a matter of style. In an interesting 2010 blog post that touched on this topic, one writer quoted from the style guide of the New York Times:
Use apostrophes for plurals of abbreviations that have capital letters and periods: M.D.’s, C.P.A.’s. Also use apostrophes for plurals formed from single letters: He received A’s and B’s on his report card. Mind your p’s and q’s.
But do not use apostrophes for plurals of abbreviations without periods, or for plurals formed from figures: TVs, PCs, DVDs; 1990s, 747s, size 7s.
Yet another reason to use an apostrophe in your particular example is to avoid confusing the plural As (meaning more than one A) with the adverb/conjunction as.
As for the general case (a "stylish" plural), that's probably best avoided most of the time, but rare exceptions exist. For example, I could see myself writing:
That's one of the do's on my do's-and-don'ts list
That might be justifiable, because the word dos looks more like the Spanish word for two than the plural of the word do (which is a word that isn't pluralized very often). However, I'd be willing to let a professional editor override that style choice.
I found one other grammarbook website that talked about some of these very same issues, under Rule 2b, Do not use an apostrophe + s to make a regular noun plural:
In special cases, such as when forming a plural of a word that is not normally a noun, some writers add an apostrophe for clarity.
Example: Here are some do's and don'ts.
In that sentence, the verb do is used as a plural noun, and the apostrophe was added because the writer felt that dos was confusing. Not all writers agree; some see no problem with dos and don'ts.
However, with single lowercase letters, it is advisable to use apostrophes.
Example: My a's look like u's.
The website goes on to say, "Imagine the confusion if you wrote that sentence without apostrophes. Readers would see My as look like us, and feel lost."