6. A 5-digit natural number $a b c d e$ is called a “ $\pi_{1}$ " number if and only if it satisfies $ad$ $>_{e}$ (such as 12430, 13531, etc.), then the number of “convex” numbers among all 5-digit numbers is ( ).
A. 8568
B. 2142
C. 2139
D. 1134
$\begin{array}{l}\text { 6. B } \\ \text { Divide into 7 categories according to } \tau=3,4,5,6,7,8,9 \text {, there are in total: } C_{2}^{2} C_{3}^{2}+C_{3}^{2} C_{4}^{2}+C_{4}^{2} C_{5}^{2}+\cdots+ \\ C_{8}^{2} C_{9}^{2}=2142 \text { . }\end{array}$