交换机完成一次换向操作，通常要经历三个过程：（）、空气和废弃的交换、开煤气。
参考答案：关煤气
偷工减料引起产品事故属于风险因素中的______。
You claim that（）travelling by boat I am wasting part of my holiday: on the contrary. I regard the sea journey as the most enjoyable part of it.
A. by
B. in
C. since
D. over
