单项选择题
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
A．a machine using solar energy efficiently has already been developed B．the sun is our final source of power C．man uses only a tiny part of the sun's energy D．the part of the sun's energy we use is used indirectly