Add parentheses to the expression so that its value is 6:

3 + 9 ÷ 9 - 6


A. (3 + 9) ÷ 9 - 6

B. 3 + (9 ÷ 9) - 6

C. 3 + 9 ÷ (9 - 6)

D. No parentheses are needed.

Please explain why we need parentheses and when we know to use them.