题干

已知玉米高秆(D)对矮秆(d)为显性,抗病(R)对易感病(r)为显性,控制上述性状的基因位于两对同源染色体上.现用两个纯种的玉米品种甲(DDRR)和乙(ddrr)杂交得F1,再用F1与玉米丙杂交(图1),结果如图2所示,分析玉米丙的基因型为(  )

A:DdRr 

B:ddRR 

C:ddRr

D:Ddrr

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C

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