A particle is moving eastwards with a velocity of 5 m/s. In 10 s, the velocity changes to 5 m/s northwards. The average acceleration in this time is
(a) zero
(b) 1/√2 m s-2 towards north-west
(c) 1/√2 m s-2 towards north-east
(d) 1/√2 m s-2 towards north