12. Show that if g is a cr-algebra, then 3 n B = {F n B:F e 9'} is a cr-algcbra. 13. Show that 13(9) = 0, P(A) 5 1, P(A) + PM\") = 1, and that if A C B, then P(A) 5 P(B). 14. If P(A) = 1/3 and P0?) = 1/4, can A and B be disjoint