Let N and Z denote the set of all natural numbers and the set of all integers respectively. The function $$f : N \rightarrow Z$$ defined by $$f(n) = \frac{n}{2}$$ or $$-\left(\frac{n - 1}{2}\right)$$, according as n is even or odd, is

A

injection only

B

srujection only

C

bijection only

D

neither an injection nor a surjection