Chọn đáp án phù hợp nhất để hoàn thành đoạn văn sau

About twenty percent of the world's present energy already come from the sun in one form or another. Special devices have already been ___(1)___ effective to place on the roofs of houses and flats to collect the sun’s rays. Thousands of these devices are now being used to provide ___(2)___ in homes throughtout the United States ___(3)___ more than a million solar water-heating units have already been built in home ___(4)___ Japan. Other purposes for which solar energy is at present being used include the ___(5)___ of salt from seawater, irrgation and sewage disposal.
For most people in developing countries, the need is not for air conditioners or central heating but for cheap ways of cooking food, drying crops and lighting homes.