答案为C项。

　　7．What can we infer from the text?

　　A．It's important for travelers to understand cultural differences.

　　B．It's useful for travelers to know how to ask the way properly.

　　C．People have similar understandings of politeness.

　　D．New Yorkers are generally friendly to visitors.

　　答案与解析：A　推理判断题。根据文章第一段及整个文章的发展脉络分析：选项C、D显然是文章的细节，而选项B只说明了游客问问题的合适方式而没有提到回答的文化背景，因此，选项A更能涵盖文章的主要内容。

　　C

　　Drying food is a simple, low­cost way to keep food safe for eating. A piece of equipment that uses the sun is a good way to dry food. The simplest solar dryer is called the direct dryer.It has a cover made of clear glass or plastic. When the sun shines into the box，heat is produced. The heat is trapped inside the box and cannot escape back through the cover. The heat dries the food. The solar dryer works better if the sides of the box are black. This is because dark colors hold heat while light colors reflect it. One way to make the sides black is to use wood that has been blackened by fire.

　　The box can be made of almost any material such as wood and metal. The surface on which the food is placed should permit air to enter from below and pass through to the food. A surface made of wires with small square openings works very well. You should use wire with the largest openings that do not allow the food to fall through. Air that comes in from below the wire surface will also carry away water evaporated(蒸发)from the food as it dries.

　　A direct dryer will dry most vegetables in two­and­one­half to four hours at temperatures from 43℃ to 63℃. Fruits take longer，from four to six hours at the same temperatures. Solar food drying is fast，safe and low­cost. Solar dried food tastes delicious and looks good. It is also healthy because nutrients such as vitamin C are kept in the food.

8．We learn from the text that the solar dryer works well when ________.