A function $f$ from the set of natural numbers of integers defined by <br> $f(n) = \begin{cases} \frac{n-1}{2}, & \quad \text{when } n \text{ is odd}\\ -\frac{n}{2 } & \quad \text{when } n \text{ is even} \end{cases}$ <br> is :

Question

( A ) onto but not one-one
( B ) one-one and onto both
( C ) neither one-one nor onto
( D ) one-one but not onto