A function $f:N\to Z$ defined by

$f\left(n\right)=\left\{\begin{array}{l}\frac{n-1}{2},\text{\hspace{0.33em}when\hspace{0.33em}'n'\hspace{0.33em}is\hspace{0.33em}odd}\\ \frac{-n}{2},\text{\hspace{0.33em}when\hspace{0.33em}'n'\hspace{0.33em}is\hspace{0.33em}even}\end{array}\right.,$ is

• A

  one-one but not onto

• B

  onto but not one-one

• C

  one-one onto

• D

  neither one-one nor onto