企业物流系统构成的垂直结构包括（）、（）和（）。
参考答案：管理层；控制层；作业层
— Could you please tell us about some good ways to learn English?
— My suggestion is that reading often and writing regularly _____. [ ]
A. are needed
B. is needed
C. be needed
D. should need
封堵方式和封堵技术？
