在平面直角坐标系xoy中，已知△ABC顶点A（-4，0）和C（4，0），顶点B在椭圆$\frac{x^2}{25}+\frac{y^2}{9}=1$上，则$\frac{\sin A+\sin C}{\sin B}$=                   （  ）

— Talking loudly in a library is impolite.

— In fact, we should also ________ not to cough(咳嗽) or sneeze（打喷嚏）loudly in public.

Are these things good for our____(膝盖)?