一、选择题(以下各小题给出的四个选项只有一个是正确的,把正确选项前面的字母填在括号内.每小题3分,共12分)1.在铁路道口常设有如图1所示的道闸,这种道闸平时上扬,当火车要通过道口时道闸转到水平位置,示意过往的其他车辆停车等候,这种道闸称为常开型 First, multiple-choice questions (only one of the four options given in the following sub-questions is correct. Put the letters in front of the correct options in parentheses. Each sub-question 3 points, a total of 12 points) 1. In the railway crossing often has a figure Gates shown in Fig. 1 are usually raised. When the train is passing through a crossing, the gate is turned to a horizontal position, indicating that other vehicles in the past are waiting to be parked. Such a gate is called a normally open type.