乳糖操纵子中，结合RNA聚合酶的DNA序列是
A．调节基因
B．启动子
C．操纵基因
D．结构基因
参考答案：B
You should understand the math problem now, you have had it ______ three times. [ ]
A. explaining
B. to explain
C. explained
D. explain
目前，基本养老保险中，个人账户的规模为本人缴费工资的（）。
A.8%
B.10%
C.15%
D.20%
