问题
选择题
It is often said that an American starts a speech with a joke , _______ a Japanese has an apology to make.
A．which
B．what
C．while
D．that
答案
答案：C
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It is often said that an American starts a speech with a joke , _______ a Japanese has an apology to make.
A．which
B．what
C．while
D．that
答案：C
聚氯乙烯（主要含有C、H、Cl三种元素）薄膜在使用中会分解出对人体有害的物质，故不能用于盛放和包装食品．装食品的塑料袋应是聚乙烯（主要含有C、H两种元素）制作的．鉴别聚氯乙烯和聚乙烯最简单、最实用的方法是燃烧法，根据燃烧时产生的现象和气味不同来鉴别．（1）请同学们取一小片聚氯乙烯制作的塑料和一小片聚乙烯制作的塑料，用镊子夹住，分别放在酒精灯火焰上烧，待一会儿后，远离火焰，注意观察实验时产生的现象和气味，并记录下来．
| 聚氯乙烯 | 聚乙烯 | |
| 实验现象 |