题目

阅读下面短文，从短文后各题所给的四个选项(A, B, C 和D)中选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。 Maggie was assigned to a public school in the middle of the year, and the headmaster asked her to teach Class 4-B right away. She heard that the former teacher had 1 suddenly, but the headmaster didn't tell her 2. All he told her was that this was a class of “3 students”. First day, she walked into the classroom, spitballs (纸团) 4 through the air, feet on desks, the noise deafening. She walked to the front of the classroom and 已知一批豌豆种子中胚的基因型为AA与Aa的种子数之比为1∶2，将这批种子种下，自然状态下（假设结实率相同）其子一代中胚的基因型为AA、Aa、aa的种子数之比为 （ ）A．3∶2∶1 B．4∶4∶1C．3∶5∶1 D．1∶2∶1