从核酸链的末端把核苷酸依次水解下来的酶是（）
A.剪接体
B.核酸外切酶
C.RNA聚合酶
D.DNA连接酶
E.拓扑异构酶
参考答案：B
. The man is said to have worked as a spy in the s, when he ____ information for the government.
A．was gathered
B．gathered
C．had gathered
D．has gathered
竣工文件中，绘制系统原理图时，应注意它与竣工文件的哪些项目有关联？
