JEE Math Practice Question

Let a function f : N $$\to$$ N be defined by $$f(n) = \left[ {\begin{matrix} {2n,} & {n = 2,4,6,8,......} \\ {n - 1,} & {n = 3,7,11,15,......} \\ {{{n + 1} \over 2},} & {n = 1,5,9,13,......} \\ \end{matrix} } \right.$$ then, f is

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