DNA连接酶
A．去除引物，填补空缺B．使双螺旋DNA链缺口的两个末端连接C．解除DNA旋转引起的打结、缠绕D．将双螺旋解链，便于复制
参考答案：B
Finding the old man in danger , . [ ]
A. a doctor was sent for without delay.
B. an ambulance was called for at once.
C. his son sent for a doctor at once.
D. The house was crowded with his children.
从所给的四个选项中，选择最合适的一个填入问号处，使之呈现一定的规律性。（）
A.选项A
B.选项B
C.选项C
D.选项D
old man in danger , . [ ]