灰度是如何实现的?
参考答案：
LED显示屏的亮度控制或称灰度是通过调整单位时间内的电亮时间长短实现的，又称为脉宽调制或占空比调灰
Questions 17 to 20 are based on the following passage. At the end of the passage, you will be given 20 seconds to answer the questions. Now listen to the passage.
Why was the speed limit first reduced to 55 miles-per-hour
A．The older people wanted it.
B．The voters decided on it.
C．It was decided that speeds above that were not safe.
D．The US had a fuel crisis.
混凝剂的重力投加方式的适用条件和设计要求包括( )。
A．适用于中小水厂
B．吸水管路和水泵进行防腐
C．应设防止空气进入吸水管措施
D．药投于水泵吸水管或吸水井中吸水喇叭口处，利用水泵叶轮混合。取水泵房离水厂加药间较近时采用
