JEE Math Practice Question

A function $$f$$ from the set of natural numbers to integers defined by $$$f\left( n \right) = \left\{ {\begin{matrix} {{{n - 1} \over 2},\,when\,n\,is\,odd} \\ { - {n \over 2},\,when\,n\,is\,even} \\ \end{matrix} } \right.$$$ is

1. A.neither one -one nor onto
2. B.one-one but not onto
3. C.onto but not one-one
4. D.one-one and onto both