When a Noun Ends in "O" and Is Preceded by a Vowel

When a noun ends in "o" and is preceded by a vowel (e.g. studio) , its plural is formed by adding:

A) an apostrophe and an "es" at the end of the word.
B) an apostrophe and an "s" at the end of the word.
C) only an apostrophe at the end of the word.
D) only an "s" at the end of the word.