-- Mum, can I go to the zoo with Jack?
-- When your homework , you can.
A:is done
B:was done
C:has been done
如图,两同心圆的圆心为O,大圆的弦AB切小圆于P,两圆的半径分别为6、3,则图中阴影部分的面积是____ .
膝跳反射的神经中枢是( )