—I phoned you at 7:00 this morning. But there was no answer.

—Well, maybe I _________ morning exercises at that time.

A．was doing

B．did

C．is doing

D．would do

已知豌豆子叶黄色(Y)对绿色(y)是显性，种子形状圆粒(R)对皱粒(r)是显性，植株高茎(D)对矮茎(d)是显性，三对基因分别位于不同的同源染色体上。现有甲、乙、丙、丁四个品系的纯种豌豆，其基因型如下表所示。

若用杂交育种的方式培育出yyrrdd新类型，至少应选用上述品系中的（）作育种材料。