填空题

Almost all energy on Earth comes from the sun. Even the fossil fuels (such as coal and oil) were produced originally by the heat of the sun. Unlike these non-renewable fuels, the sun’s energy is (1) . It might sometimes seem as if the sun doesn’t come out very much. Yet if we could learn to trap all the sunlight that falls on Earth in just one hour, we would have enough energy to (2) the whole world with fuel for a year. So far, scientists have not discovered any way of doing this. We must still get our energy from (3) fossil fuels and wood. We can make use of the rivers to make hydroelectricity, and (4) we can use direct solar power — that is, the sun’s heat. In the (5) countries, people use animal power. And, of course, we have also (6) nuclear power.