A particle starting from rest has a constant acceleration of `4 m//s^2` for 4 s. It then retards uniformly for next 8 s and comes to rest. Find during the motion of particle (a) average acceleration (b) average speed and (c) average velocity.
A. `4m//s^(2)`
B. zero
C. `8 m//s^(2)`
D. `-4m//s^(2)`