A particle is moving eastwards with a velocity of $5 \mathrm{~m}/\mathrm{s}$ in 10 seconds the velocity changes to $5 \mathrm{~m}/\mathrm{s}$ northwards. The average acceleration in this time is\\
(1) $\frac{1}{\sqrt{2}} \mathrm{~m}/\mathrm{s}^2$ towards north-east\\
(2) $\frac{1}{2} \mathrm{~m}/\mathrm{s}^2$ towards north.\\
(3) zero\\
(4) $\frac{1}{\sqrt{2}} \mathrm{~m}/\mathrm{s}^2$ towards north-west