$6 \cdot 11 n$ is a positive integer, if $a^{2 n}=5$, then the value of $2 a^{6 n}-4$ is:
(A) 26.
(B) 246.
(C) 242.
(D) $12 \sqrt{5}-4$.
(China Beijing Junior High School Mathematics Competition, 1982)
[Solution] $2 a^{6 n}-4=2 a^{2 n \cdot 3}-4=2\left(a^{2 n}\right)^{3}-4=2 \cdot 5^{3}-4=246$. Therefore, the answer is $(B)$.