8. What is the last digit of $7^{\left(7^{7}\right)}$ ?
(A) 1 ;
(B) 3 ;
(C) 5 ;
(D) 7 ;
(E) 9 .
8. Ans: (B)
The last digit of $7^{k}$ is $1,7,9,3$ respectively for $k \equiv 0,1,2,3(\bmod 4)$. Since $7^{7} \equiv(-1)^{7} \equiv 3(\bmod 4)$, the last digit of $7^{\left(7^{7}\right)}$ is 3 .