$1.5^{1000}$ 's last three digits are ( ).
(A) 025
(B) 125
(C) 625
(D) 825
-、1. B.
If $n \geqslant 3$, then when $n$ is odd, the last three digits of $5^{n}$ are 125. Hint: Discuss the cases when $n \geqslant 3$, $n$ is odd, and $n$ is even.