— I wonder _______.
— I'm afraid we'll be late.
A:how we can be on time
B:if we arrive at the meeting on time
C:why we get to school late
D:if we will arrive at the meeting on time
抛物线有光学性质:由其焦点射出的光线经抛物线折射后,沿平行于抛物线对称轴的方向射出。现已知抛物线y2=2px(p>0)的焦点为F,过抛物线上点P(x0,y0)的切线为l,过P点作平行于x轴的直线m,过焦点F作平行于l的直线交m于M,则PM的长为( )
该图为我国的传统民居窑洞,它是属于以下哪个地形区的民居景观( )
用化学符号或化学用语填空.