There's a bird _______in the tree.
A:sing
B:sings
C:singing
D:to sing
(Ⅰ)求A∩B,(∁RB)∪A;
(Ⅱ)若C⊆A,求实数a的取值范围.
如图,AB为⊙O的切线,AC、BD分别与⊙O切于C、D点,若AB=5,AC=3,则BD的长是____
下列说法中正确的是( )