Using Apostrophes

Indicating omission of letters.
Apostrophes form the contractions. Its function is to show that some letter(s) is/ are omitted. Contracted forms are accepted in oral speech and informal writing. To form a contraction, put an apostrophe in the place of omitted letter(s) Look at these examples:

• doesn't = does not
• You're = you are
• She'll = she will
• What's = what is
• Mightn't = might not
• Hasn't = has not
• I'd = I would / I had
• '70 = 1970

Making up plurals of lowercase letters.
To make up the plural of a lowercase letter, put 's after the letter. Apostrophes are not necessary with capitalized letters, numbers, and symbols (however mind that some tutors or professors will expect you to use them).

According to American Heritage - Book of English Usage "with lowercase letters, symbols, abbreviations with periods, and in cases where confusion might arise without an apostrophe, use -'s to form the plural: p's and q's; +'s; -'s; M.A.'s; A's and I's; 2's. Mainly your goal is to be as clear as possible and avoid confusion".