$4.1997^{2000}$ when divided by 7 leaves a remainder of ( ).
(A) 1
(B) 2
(C) 4
(D) 6
4. C
4. From $1997=7 n \div 2$, we have $1997^{3}=(7 n+2)^{3}=7 m + 1$, thus $1997^{1998}=7 k+1$. Therefore, $1997^{2000}=(7 k+1)(7 h + 4)$. ($m, n, k, h$ are integers).