The author's attitude toward the future of solar energy is ______.

A．The technical problem of providing heat on cloudy days are not yet solved.
B．It costs much to have a solar energy system installed at home.
C．There are basically two types of solar energy.
D．Solar energy is clean, inexhaustible and free of risk.

A．marketing of software
B．one version of the movie Close Encounters of the Third Kind
C．major computer companies
D．impact of TV commercials