18. Let $a=1+3^{1}+3^{2}+3^{3}+3^{4}+\ldots+3^{999}+3^{1000}$, then the remainder when $a$ is divided by 4 is ( ).
(A) 3.
(B) 2.
(C) 1.
(D) 0.
Answer: $\mathbf{C}$.
Solution: Since $a=1+3(1+3)+3^{3}(1+3)+3^{5}(1+3)+\cdots+3^{999}(1+3)$, the remainder when $a$ is divided by 4 is 1. Therefore, the correct choice is C.