大家都著重在他是名詞或複合形容詞,可是對我來說,重點是標準格式單位的縮寫,本來複數就不加s 順手查了幾個ref
https://goo.gl/P1g1QD "The letter "s" is never added toa symbol to indicate a plural. In other words, 2 minutesis written 2 min, not 2 min. or 2 mins."
https://goo.gl/QiiTJy "There is no need to add an s toan abbreviation to show a plural"例如長度也是 5 cm, 3 mi, 小時也是 3 h 而不是 3 hsBBC那個只是剛好他也是複合形容詞的一部份所以本來就單數型