6. Let a1,a2,a3,a4 be any permutation of 1,2,3,4, and f be a one-to-one mapping from {1,2,3,4} to {1,2,3,4} such that f(i)=i. Consider the matrix
A=[a1f(a1)a2f(a2)a3f(a3)a4f(a4)].
If the corresponding positions of two matrices M and N differ in at least one entry, then M and N are considered two different matrices. The number of different matrices that satisfy the conditions is ( ).
(A) 144
(B) 192
(C) 216
(D) 576