21.   The renewable energy sources must eventually dominate world energy supply.

22.   The seasonal performance might be improved to some extent by using a wind turbine as the energy source.

23.   The sun is perhaps the least expensive energy source we have.

24.   The ultimate aim is to replace gasoline altogether by using battery power or other non-polluting energy sources.

25.   The ultimate aim is to replace gasoline with non-polluting energy sources.

26.   These discouraging factors combine to compel a search for cheaper and environmentally safer energy sources.

27.   We hope to see increased usage of renewable energy sources.

28.   Scientists are trying to find ways of reducing our dependence on energy sources that contribute towards global warming.

29.   We should conserve oil and gas by making full use of other energy sources.