4.7 As shown in the figure:

Let the rational numbers corresponding to points $A, B$ on the number line be $a, b$, respectively, and let $x=\frac{a-5 b}{a+5 b}$. Then, which of the following inequalities is correct?
(A) $0<1$.
(C) $-1<x<0$.
(D) $x<-1$.
(8th "Jinyun Cup" Junior High School Mathematics Invitational, 1991)
[Solution] From the graph, we can see: $-21, \quad \therefore \quad x<-1$. Therefore, the correct choice is $(D)$.