题目

Western civilization has been deeply influenced by Greek Culture. Many words from Greek mythology (神话) contributed to English vocabulary today. Greek mythology tells of the myths of ancient Greece, concerning its gods，heroes and the nature of the world. It attempts to explain the origin of the world and the adventures and lives of the gods, heroes and natural creatures. The following words exactly come from Greek mythology. Echo; Narcissus Echo was a fairy (仙女). She got the punishment of saying nothing but repeat others, last sentence said to her because she kept talking and 将同种黄色鼠（A）和灰色鼠（a）杂交，已知A和a是由常染色体上的一对等位基因控制的。有一位遗传学家在实验中发现含显性基因（A）的精子和含显性基因（A）的卵细胞不能结合。如果黄色鼠与黄色鼠（第一代）交配得到第二代，第二代老鼠自由交配一次得到第三代，那么在第三代中黄色鼠的比例是 A．1                 B．4/9              C．1/2               D．5/9