---Nowadays children are bearing too heavy a learning burden.

---______. So much homework do they have to do that they almost have no time for other activities.

A．So what?

B．It cannot be better

C．Good idea

D．I cannot agree more

抛物线与y轴的交点为（0，4），与x轴的交点为（-1，0）和（2，0），则抛物线的函数关系式为______．