原点和点(1,1)在直线x+y-a=0的两侧,则a的取值范围是( )
A:a<0或a>2
B:a=0或a=2
C:0<a<2
D:0≤a≤2
— John, could you please the black-board now? It's your duty today.— OK. I'll do it right away.