The function f is defined by f(x) = ~$-\frac{1}{x}$~  for all nonzero numbers x.  If f(a) = ~$-\frac{1}{2}$~  and f(ab) = ~$\frac{1}{6}$~ , then b =

3

~$\frac{1}{3}$~

~$-\frac{1}{3}$~

-3

-12