A:a是N极,b是S极
B:a是S极,b是N极
C:a、b都是N极
D:a、b都是S极
——Is Dongting Lake the largest lake in China?——No, it's the second ____ one ____ all the lakes in China.
若有二次函数y=ax2+c,当x取x1,x2(x1≠x2)时,函数值相等,则当x=x1+x2时,函数值为( )
哺乳动物起源于