8.7 The lengths of seven line segments $a_{1}, a_{2}, \cdots$, $a_{7}$ are all integers, satisfying $a_{1}<a_{2}<\cdots<a_{7}$, and no three of these seven line segments can form a triangle. If $a_{1}=1, a_{7}=$ 21, then $a_{6}=(\quad)$.
(A) 18
(B) 13
(C) 8
(D) 5
8.B.

Only when $a_{2}=2, a_{3}=a_{1}+a_{2}=3, a_{4}=$ $a_{2}+a_{3}=5, a_{5}=a_{3}+a_{4}=8, a_{6}=a_{4}+a_{5}=$ 13, none of the 7 segments can form a triangle with any other two, so $a_{6}=13$.