查阅《化学手册》知，Al、Al₂O₃、Fe、Fe₂O₃熔点、沸点数据如下：

下面关于音调和响度的说法中正确的是（    ）

归纳是学习科学的重要方法，总结出下图所示的物质之间的反应关系(图中短线连接的物质表示能相互反应)，请帮助完成下列空白：

Different countries have different customs. When you travel to another countries，please follow their customs，just as the saying goes，“             .”Very often people who travel to the United States forget to tip（付小费）.It is usual to tip porters who help carry your bags，taxi drivers and waiters. Waiters expect to get a 15% tip on the cost of your meal. Taxi drivers expect about the same amount.
In England，make sure to stand in line even if there are only two of you. It's important to respect lines there. It’s a good idea to talk about the weather. It's a favorite subject of conversation with the British.
   In Spain，it's a good idea to have a light meal in the afternoon if someone invites you for dinner. People have dinner very late，and restaurants do not generally open until after 9 pm.
   In Arab countries，men kiss one another on the cheek（脸颊）. Your host may welcome you with a kiss on both cheeks. It is polite for you to do the same.
   In Japan，people usually give personal or business cards to each other when they meet for the first time. When a person gives you a card，don't put it into your pocket right away. The person expects you to read it.Don't forget to be careful of your body language to express something In conversation. A kind of body language that is acceptable in one culture may be impolite in another.

一个富裕的商人，常年从事丝绸生意，为了多赚钱，他雇用了几十个工人为他生产布匹。这一现象最早出现在(    )