我国南方地区重要的粮食作物是( )
如图,AB,CD是⊙O的直径,点E在AB延长线上,FE⊥AB,BE=EF=2,FE的延长线交CD延长线于点G,DG=GE=3,连接FD.
(1)求⊙O的半径;
(2)求证:DF是⊙O的切线.
Can you stand on the s____ of the planet Mars without wearing special boots?