5. The number of different four-digit numbers that can be formed using the eight digits $1,1,2,2,3,3,4,4$ is ( ).
(A) 204
(B) 144
(C) 72
(D) 24
5. A

There are three cases:
(1) Four-digit numbers with no repeated digits: $P_{4}^{4}=24$ (ways).
(2) One pair of digits is the same: $C_{4}^{1} \cdot C_{3}^{2} \cdot C_{4}^{2} \cdot P_{2}^{2}=144$.
(3) Two pairs of digits are the same: $C_{4}^{2} \cdot C_{4}^{2}=36$.