题目

—Can we run across the road now?—No, we _      . We have to wait until the light turns green.    A. needn't     B. mustn't       C. couldn't       D. shouldn't 答案：B右图为基因表达载体的模式图。下列叙述正确的是A．启动子是RNA聚合酶识别和结合的部位B．终止子是DNA聚合酶识别和结合的部位C．抗生素抗性基因起鉴别和筛选作用D．限制酶切割位点供外源基因插入其中