$6 \cdot 6$ using the digits $1,9,8,5$ to form a number in the shape of $a^{b^{b^{d}}}$, if you want to maximize its value, then the value of $c$ is
(A) 1 .
(B) 9 .
(C) 8 .
(D) 5 .
(China Shanghai High School Mathematics Competition, 1985)
[Solution] It is easy to deduce that $5^{8}>8^{5}, 8^{9}>9^{8}$, etc. Thus, $5^{8^{9^{1}}}$ is the number with the largest value, i.e., $c=9$.

Therefore, the answer is $(B)$.