单项选择题
A．100 % but indirectly B．5 % C．50 % D．only a very tiny amount
A．harnessing energy through chemistry B．our last source of power C．how water power is produced D．the amount of solar energy that can be utilized
A．only a very small part is available B．the sun is too distant from the earth C．it takes thousands of years for this energy to be stored up in coal and oil D．our scientific knowledge is not sufficient