British people say "please" and "thank you" all ______ time, and they always keep their voice down in ______ public.
某同学用托盘天平和量筒测量一小石块的密度,图甲是调节天平时的情形,图乙和图丙分别是测量石块质量和体积时的情形,下列说法错误的是( )
已知抛物线y2=4x的焦点为F,圆C:x2+(y﹣5)2=r2与该抛物线交于A,B两点,若A、B、F三点共线,则AB的长度为( )