A particle is moving eastwards with velocity of $5 \mathrm{~m} / \mathrm{s}$. In $10 \mathrm{sec}$ the velocity changes to $5 \mathrm{~m} / \mathrm{s}$ northwards. The average acceleration in this time is
(a) Zero
(b) $\frac{1}{\sqrt{2}} \mathrm{~m} / \mathrm{s}^{2}$ toward north-west
(c) $\frac{1}{\sqrt{2}} \mathrm{~m} / \mathrm{s}^{2}$ toward north-east
(d) $\frac{1}{2} \mathrm{~m} / \mathrm{s}^{2}$ toward north-west