如图,AB为⊙O的弦,OC⊥AB于C,AB=8,OC=3,则⊙O的半径长为( )
A:7
B:3
C:4
D:5
If you do things ________, usually you can't do them well.
一组数据1,x,4,y,5的中位数和平均数都是3(x<y),则x=____.这组数据的方差是____.
自来水厂净水过程中用到活性炭,其作用是____.