预紧密封比压
参考答案：
预紧时，迫使垫片变形与压紧面密合，以形成初始密封条件，单位面积上所需的最小压紧力。称为预紧密封比压。
The Education Department says that school safety has set off alarm bells with frequent reports of serious accidents ________ students got injured or killed.
A．after which
B．in which
C．by which
D．for which
现有①石油的分馏 ②煤的干馏 ③油脂的硬化 ④蛋白质的盐析 ⑤蛋白质的变性⑥橡胶的老化。其中属于物理变化的是
A．①②④
B．①④
C．③④⑥
D．①③⑥
