$5.1003^{2000}$ 's last digit is ( ).
(A) 1
(B) 3
(C) 7
(D) 9
5.D.

It is easy to know that the last digit of $1003^{n}$ is the same as that of $3^{n}$. The last digits of $3^{4 k}, 3^{4 k+1}$, $3^{4 k+2}, 3^{4 k+3}$ are $1,3,9,7$, and since 2006 is a number of the form $4 n+2$, the solution follows.