A、The United States. B、DPRK. C、Afghanistan. D、Uganda. C细节题。文章开头提到，早在30多年前Afghanistan就开始研制thedevice，联系上句可知the device指的就是solar wat

Solar water heating is used worldwide. The device we are about to describe was developed in Afghanistan more than 30 years ago. It can heat 70 litres of water to 60 degrees Celsius. It can do this between sunrise and noon on a clear day with an average outside temperature of 32 degrees Celsius.
    The solar water heater has two parts. One part is the solar collector. This is made of metal sheets. The collector is placed in contact with the water. There are several kinds of metal sheets that can be used for the collector. Metal sheets that have raised sections will work very well. Once the water is heated, it is kept hot with special material. This helps the water stay warm for a long time. The second part of the solar water heater is the storage tank. Two rubber pipes are attached to the storage tank. One pipe lets water flow into the system. The other lets water flow out. When the water heater is working correctly, water will flow from the storage tank to the collector and back again. You can use the hot water at the top of the tank for washing and cleaning.
    You can change the flow of water so that the temperature is hot or warm as desired. This solar water heater is easy to build and operate. It will last about two years before the rubber pipes need to be replaced. However, it will heat water only on sunny days.
16. In which country was solar water heating developed?
17. What is a solar water heater composed of?
18. How can people change the temperature of water from a solar water heater?