quEstions rEFEr to thE Following grAph.

iF EvEry stuDEnt who wAs in thE ElEvEnth grADE in 1996 AnD who usED A ComputEr in sChool At lEAst onCE A wEEk usED A ComputEr At lEAst onCE A wEEk in 1997, AnD iF 20% oF thosE stuDEnts who wErE in thE ElEvEnth grADE in 1996 AnD who DiD not usE A ComputEr in sChool At lEAst onCE A wEEk, DiD usE A ComputEr in sChool onCE A wEEk in 1997, thEn whAt pErCEnt oF twElFth-grADErs in 1997 usED A ComputEr in sChool At lEAst onCE A wEEkA．40%
B．50%
C．60%
D．70%
E、80%