Quick: if you want to refer to the decade in which the moon landing occurred, do you write (a) 1960’s, or (b) 1960s? How about if you’re selling some vintage records at a garage sale? Does your sign read (a) LP’s for Sale, or (b) LPs for Sale?
If you answered “b” to both, congratulations. You’re correct, and you’re dismissed.
If you answered “a” to either one, you probably read the New York Times, so it’s not entirely your fault. But you’re both wrong.
