Realize Four Transitions 实现四个转变
Britain has four seasons, and while the transitions between them can be pleasant, each is typically as wet, gray and cold as the last.
英国有四季，虽然四季交替时比较舒适，但每一季都很分明：潮湿、灰暗、寒冷。
The events causing state transitions are classified as four classes: System Event, which includes data communication and command from teacher's station;
把引发状态转换的事件分为四类：数据通讯、教师台操作等系统事件；
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