列管式换热器按结构可分为（）、（）、（）、（）。
参考答案：固定管式；浮头式；填料式；“U”形管式
在列车允许速度小于120km/h地段进行改道作业，列车通过时允许每隔（）枕木头有一根木枕不钉道钉。
A.1根
B.2根
C.6根
D.7根
— I've been working really hard at English recently, but it still remains poor.
— Don't give up. Sometimes things don't _____ like you want right away. [ ]
A. turn out
B. turn up
C. turn down
D. turn in
