A:(0,1)
B:(1,2)
C:(2,3)
D:(3,4)
Volunteering our time to help people ________ a good way to _______our free time.
如图所示,把甲、乙两个体积相同的实心小球投入水中,甲球漂浮,乙球悬浮.则下列说法正确的是( )