A particle moving eastwards with $5 \mathrm{~m} / \mathrm{s}^{-1}$. In $10 \mathrm{~s}$ the velocity changes to $5 \mathrm{~m} / \mathrm{s}^{-1}$ northwards. The average acceleration in this time is
(A) $\frac{1}{\sqrt{2}} \mathrm{~m} / \mathrm{s}^{-2}$ towards Northeast
(B) $\frac{1}{2} \mathrm{~m} / \mathrm{s}^{-2}$ towards North
(C) $\frac{1}{\sqrt{2}} \mathrm{~m} / \mathrm{s}^{-2}$ towards Northwest
(D) Zero