4. What is the unit digit of $3^{\left(8^{7}\right)}$?
(A) 1
$\begin{array}{ll}\text { (B) } 7 & \text { (C) } 3\end{array}$
(D) 9
(E) 5
Problem 4. The answer is (A).

Since $3^{4}=81$, every power $3^{4 n}=\left(3^{4}\right)^{n}$ ends with the digit 1. The number $8^{7}$ is certainly a multiple of 4, therefore the units digit of $3^{\left(8^{7}\right)}$ is 1.

(Problem proposed by S. Yang)