Read the passage and choose A, B, C or D to complete the blanks:

       The Sun has provided us with many uses. Thousands of years ago, people began to dry clothes and food in the sunlight. But it’s not until recently that we have been able to generate power from it. There are three main ways (19) ______ using the Sun’s energy: solar cells, solar water heating and solar furnaces. One of good points of solar energy is that it is an unlimited (20) ______. Moreover, it produces no waste or pollution (21) ______is one of the         most serious problems in the world. Therefore, in countries with sunny climate, solar energy has great potential. (22) ______, it has some disadvantages as well such as: it doesn't work at night, and the cost is very high because it is expensive to build solar power stations.