I love this song by Lady Gaga. Would you ________ the TV a bit, please? I can't hear it clearly.
A:turn on
B:turn off
C:turn up
D:turn down
如图,PA、PB是⊙O的两条切线,A、B是切点,若∠APB=60°,PO=2,则⊙O的半径等于____.