Let M={7,8,9}. Determine which of the following functions is invertible for f:M→M.

• A. f = {(7,7),(8,8),(9,9)}
• B. f = {(7,8),(7,9),(8,9)}
• C. f = {(8,8),(8,7),(9,8)}
• D. f = {(9,7),(9,8),(9,9)}