题目

―You didn’t wait for Mr. Smith last night, did you?―Yes, but we          . He didn’t return home at all.       A．don’t have to       B．needn’t have        C．didn’t need to      D．should have 答案：B1．基因型为AaBbCc（基因自由组合）的个体中，产生ABc配子的比例为（　　）A．$\frac{1}{2}$B．$\frac{1}{4}$C．$\frac{1}{8}$D．$\frac{9}{16}$