A particle moves along the positive branch of the curve $y=\frac{x^{2}}{2}$ where $x=\frac{t^{2}}{2}$, where $x$ and $y$ are measured in metre and $t$ in second. At $t=2 \sec$, the velocity of the particle is:
(a) $(2 \hat{1}-4 \hat{j}) \mathrm{m} / \mathrm{sec}$
(b) $\left.\left(2 \hat{1}+4^{2}\right)\right) \mathrm{m} / \mathrm{sec}$
(c) $(2 \hat{1}+2 \hat{j}) \mathrm{m} / \mathrm{sec}$
(d) $(4 \hat{1}-2 \hat{j}) \mathrm{m} / \mathrm{sec}$