6. Let $0<a<1$, if $x_{1}=a, x_{2}=a^{x_{1}}, x_{3}=a^{x_{2}}, \cdots, x_{n}=a^{x_{n-1}}$,
$\cdots$, then the sequence $\left\{x_{n}\right\}$
(A) is increasing;
(B) is decreasing;
(C) odd terms are increasing, even terms are decreasing;
(D) even terms are increasing, odd terms are decreasing.
Answer ( )
6. (C)

First, consider the first three terms of the sequence,
$$
x_{1}=a, \quad x_{2}=a^{a}, \quad x_{3}=a^{a^{a}}
$$
$$
\begin{array}{ll}
\because & 0<a<1, \\
\therefore & a<a^{a}<1 .
\end{array}
$$
$$
a<a^{a^{a}}<a^{a} \text {. }
$$

That is,
$$
x_{1}<x_{3}<x_{2} .
$$

This already shows that the sequence is neither increasing nor decreasing, and the odd terms are not decreasing either.