Which of the following function f: Z X Z ‚Üí Z is not onto?$

• A. f(a, b) = a + b
• B. f(a, b) = a
• C. f(a, b) = |b|
• D. f(a, b) = a ‚Äö√Ñ√∂‚àö√ë‚àö¬® b