6. In the permutation $a_{1}, a_{2}, a_{3}, a_{4}, a_{5}$ of $1,2,3,4,5$, the number of permutations that satisfy the conditions $a_{1}a_{3}, a_{3}a_{5}$ is ( ).
(A) 8
(B) 10
(C) 14
(D) 16
6. (D).

From $a_{1}a_{3}, a_{3}a_{3}$ we know, either $a_{2}=5$, or $a_{4}=5\left(a_{1} 、 a_{3} 、 a_{5}\right.$ cannot be the maximum), and $a_{2} \geqslant 3, a_{4} \geqslant 3, a_{3} \leqslant 3$.
(1) If $a_{2}=5$, then $a_{4}=4$ or 3.
When $a_{4}=4$, there are $3!=6$ ways;
When $a_{4}=3$, there are $2!=2$ ways.
At this time, there are a total of $6+2=8$ ways.
(2) Similarly, when $a_{4}=5$, there are also 8 ways.
Therefore, there are a total of $2 \times 8=16$ valid permutations.