4.24 If the solution set of the quadratic inequality $m x^{2}+8 m x+60<0$ is $-5<x$ $<-3$, then the value of $m$ is
(A) 1.
(B) 2.
(C) 3.
(D) 4.
(E) 5.
(Chinese Jilin Province Seven Cities and Prefectures Junior High School Mathematics Competition, 1985)
[Solution] From the given, we know that $-5, -3$ are the roots of $m x^{2}+8 m x+60=0$. According to Vieta's formulas, we have
$$
\frac{60}{m}=(-5) \cdot(-3) \text {, which gives } m=4 \text {. }
$$

Therefore, the answer is $(D)$.