我们来看看一组问答以便理解：
What is the correct way to pluralize an acronym?
缩略词复数形式的正确写法是什么？
For example, if I wanted to write the equivalent of
例如，如果我想写与下面这句话等价的话，
There are many automated teller machines in this city. 这城市里有很多自动提款机。
Would it be
是应该写成
There are many ATMs in this city.
or
还是
There are many ATM's in this city.
(could get confused with possessive form or contraction).
（会和所有格混淆，与缩略语）
or just
还是简单地这么写
There are many ATM in this city.
(assuming the final s is included in Machines represented by M).
（假定最后的s是包含在Machines 里，由“M”这个词来代表）
Maybe something else?
或者还有其他写法？
……
回答是：
The Chicago Manual of Style has an interesting way to address this: They omit the apostrophe, unless there are periods in the abbreviation. So this would give you ATMs, or alternately A.T.M.'s. (A.T.M.s looks weird.)
《芝加哥格式手册》中有一个有趣的方法来处理这种情况：他们一般省略掉单引号，除非这个缩略语中含有句点符号“.”。所以，对于你的问题，应该写作ATMs，或者用另一种写法 A.T.M.'s（而A.T.M.s看起来太奇怪了）
* 注：芝加哥格式是国外指导论文中引用条目时写法的几种规范之一。
Some web page indicates that acronyms ending in the letter "S" get an apostrophe, something I've seen before, but can't find in a general reference. So one would write ATMs and SOS's.
一些网页上指出以“S”结尾的缩略语应该加上一个单引号，我看到过，但我找不到一个通用的参考。所以你应该写ATMs 和SOS's。
This page on the North Carolina State University website references AP's rule as being to always use an apostrophe.
这个北卡州立大学的网站上引用了AP规则来指定缩略语的复数都要有一个单引号。
* 注：AP's rule指的是一种规范新闻中写法的规约。
The 2009 AP Stylebook's "plurals" entry has no section on acronyms, but mentions "VIPs", I can't find anything addressing how to specifically pluralize acronyms. (The "abbreviations and acronyms" section is also of no help.)
2009年AP风格手册中的“复数”一词没有提到缩略语的章节，但是提到过“VIPs”，我找不到如何处理特殊的缩略语复数形式的内容。（“缩写和首字母缩略语”这一节也没有帮助）
Personally, I omit using apostrophes unless I can't avoid it. I do use them when talking about single letters or where it would avoid confusion. (For example, SOs for "Significant Others" looks like an incorrectly capitalized SOS.)
我个人意见，除非无法避免的情况，我一般都省略掉单引号。在谈及单个（大写）字母时我使用（单引号），在可能必须用来避免混淆时（我也使用单引号）。（例如，“Significant Others”如果缩写成“SOs”，会被人误会成最后一个字母大小写写错了的SOS）
现在，你会选择哪一种写法呢？
