题目

39. There are hurricanes here frequently in summer. The underlined part means _____. A. often                   B. recently               C. sometimes            D. Seldom 答案：A4．一对色盲夫妇，生下了三个非色盲儿子，则可能的原因是（　　）A．基因重组B．卵细胞发生基因突变C．精子发生基因突变D．受精卵染色体数目发生变化