A particle moves in the $x y$ plane with only an $x$ -component of acceleration of $2 \mathrm{~ms}^{-2}$. The particle starts from the origin at $t=0$ with an initial velocity having an $x$ -component of $8 \mathrm{~ms}^{-1}$ and $y$ -component of $-15 \mathrm{~ms}^{-1}$. The total velocity vector at any time $t$ is
a. $[(8+2 t) \hat{i}-15 \hat{j}] \mathrm{ms}^{-1}$
b. zero
c. $2 t \hat{i}+15 \hat{j}$
d. directed along $z$ -axis