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

A

one-one and but not on to

B

onto but not one-one

C

one-one and onto both

D

neither one-one nor onto