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