B)根据句子意思，用括号中所给的单词适当形式填空。
____  ▲  (wolf) belong to the dog family, living and hunting together.
____ With the money donated, we have   ▲ (success) helped many poor young girls return to school.
____ If we always keep our worries to   ▲  (we), things will get even worse.
____ Our head teacher warned us   ▲ (not fly) kites near power lines in spring.
____ Though it seemed   ▲  (hope) to pass the exam, he still kept on trying.
____ A recent scientific report says three   ▲  (eight) of the school students suffer from sub-health(亚健康).

$\frac{4}{5}-x=\frac{2}{7}$

$x=$ ____

如图所示，可视为质点的，质量为m的小球，在半径为R的竖直放置的光滑圆形管内做圆周运动，下列有关说法中正确的是（   ）