A:studied
B:studying
C:to have studied
D:to be studying
①黑色×黑色→黑色 ②黄色×黄色→2黄色∶1黑色③黄色×黑色→1黄色∶1黑色
长江、黄河是中华民族的母亲河,读图回答下列问题.