A particle is moving castward with a velocity of $5 \mathrm{~ms}^{-1}$. If in los the velocity changes by $5 \mathrm{~ms}^{-1}$ northwards, what is the average acceleration in this timc?
(a) $1 / \sqrt{2} \mathrm{~ms}^{2} \mathrm{~N}-\mathrm{W}$
(b) $1 / 2 \mathrm{~ms}^{2} \Gamma:-\mathrm{N}$
(c) $\sqrt{2} \mathrm{~ms}^{-2} \mathrm{~N}-\mathrm{W}$
(d) $2 \sqrt{2} \mathrm{~ms}^{-2} \mathrm{~N}-\mathrm{W}$.