设双曲线$\frac{x^2}{{\alpha }^2}-\frac{y^2}{b^2}=1\left(a>0,b>0\right)$的右焦点是F，左、右顶点分别是$A_{1,}{A}_2,$过F做$A_1{A}_2$的垂线与双曲线交于B，C两点，若,则双曲线的渐近线的斜率为（    )

    “Early to bed, early to rise（起身）makes people healthy, wealthy and wise（健康，富有，聪明）.” This is an old English saying. Children should have ten hours' sleep every night, or （不然，否则）they can't do their work very well. They will not be wise. The body needs exercise. Walking, running, swimming and playing ball games are all exercise. Exercise keeps the body strong.