How many solutions are there to the equation x1 +x2 +x3 +x4 =25

where xi, i = 1, 2, 3, 4, is a non-negative integer such that

(a) xi 3 for i = 1, 2, 3, 4?

(b) x1 2,x2 3,x3 >5,x4 >7?

(c) 2

(d) 3