A force $(3 \hat{i}-2 \hat{j}) \mathrm{N}$ acting on a particle, does zero work when the particle is displaced from point $(1,-1)$ to a point $(2, a)$. Then a is (position co-ordinates are in metre)
(a) $\frac{1}{2} \mathrm{~m}$
(b) $1 \mathrm{~m}$
(c) $\frac{3}{2} \mathrm{~m}$
(d) $-\frac{3}{2}$