下列图形中具有稳定性的是( )
A:等边三角形
B:正方形
C:平行四边形
D:梯形
____个 ____个
在正四面体P﹣ABC中,D,E,F分别是AB,BC,CA的中点,下面四个结论中不成立的是( )
It's reported that there are more than 300 smokers in China, nearly a third of all the smokers in the world.