—Happy birthday!—___________
A:The same to you.
B:Thank you!
C:We are all happy.
D:Oh, we are not happy
如图,四棱锥S﹣ABCD的底面是正方形,SD⊥平面ABCD,SD=AD=a,点E是SD上的点,且DE=λa(0<λ≤1).
求证:对任意的λ=(0,1,都有AC⊥BE
下列水体中,水循环速度最快,周期最短的是( )
下列说法有错误的一项是()