与护罩方面有关的事故主要是封闭效果差、（）或护罩本身有缺陷。
参考答案：未安装护罩
下面是关于自动化生产和资本划分的一组材料，请回答：
剩余价值的含义；
When something is on fire. People throw sand (沙子) on it to put out the fire. The main use of the sand is to
________. [ ]
A. protect the thing on fire.
B. add weight to the thing on fire.
C. keep the air away from the thing on fire.
D. lower the temperature of the thing on fire.
