Two particles having position vectors \( \vec{r}_{1}=(3 \hat{i}+5 \...

Two particles having position vectors \( \vec{r}_{1}=(3 \hat{i}+5 \hat{j}) \) metres and \( \vec{r}_{2}=(-5 \hat{i}-3 \hat{j}) \) metres are moving with velocites \( \vec{v}_{1}=(4 \hat{i}+3 \hat{j}) \) \( \mathrm{ms}^{-1} \) and \( \vec{v}_{2}=(a \hat{i}+7 \hat{j}) \mathrm{m} / \mathrm{s} \). If they collide after 2 seconds, the value of \( a \) is
(A) 2
(B) 4
(C) 6
(D) 8