a$)$ By using Fibonacci series explain the difference between memoization and tabulation.

b$)$ The order of four matrices A$,$B$,$C$,$D are provided below

$50$x$40,40$x$30,30$x$20$ and $20$x$30.$ By multiplying in this order$($AB$)($CD$)$ how many different ways the $4$ matrices can be multiplied?

c$)$ Show all steps to get full points.