Consider a constraint satisfaction problem involving four variables. The initial domains of each variable are the values 1, 2 and 3. There are four constraints that must be satisfied: (1) A + B = 3, (2) A < C, (3) C = D + 1, and (4) B + D < 4.

(a) Show the steps the arc consistency algorithm goes through to find the unique solution to this problem.

(b) Now assume the initial domains of each variable include the value 0. How does this change the number of solutions to the problem?