一个操纵子通常含有

A．一个启动序列和一个编码基因

B．一个启动序列和数个编码基因

C．两个启动序列和两个编码基因

D．数个启动序列和一个编码基因

E．数个启动序列和数个编码基因

It became a key feature of the Oral Approach in the 1960s that （）.

A. new language points are introduced and practised in situations

B. accuracy is maintained

C. reading and writing are introduced later

D. translation should be avoided