How can both of the following statements be true about nuclear fusion?
1. Nuclear fusion has not been used as an energy source in the world.
The first statement is set up as a trick question; everybody know that nuclear reactors are used for energy generation around the world. However, nuclear fusion is not used in these generators. Since the artificial production of stable fusion reactions has not been demonstrated safely and consistently, nuclear fusion reactors do not exist. Instead, nuclear power plants use nuclear fission, the splitting of atoms to create heat; fusion would combine atoms to form heavier elements and waste heat, but is very difficult and much harder to control than fission.
Therefore, the first statement is correct in that man-made nuclear fusion has not yet been used as an energy source.
The second statement directly contradicts the first; if nuclear fusion has never been used as an energy source, how can it then be the primary energy source in the world? Again, this statement is set up to trick the reader; energy does not necessarily refer to electricity, but to any source of non-matter energy such as light. With this clarification, the number one source of energy in the world is obvious: the Sun, the largest self-sustaining nuclear fusion reactor in the Solar System. Now, the Sun is not the primary provider of electrical power in the world; that is still coal and oil. However, the heat and light from the Sun keeps the world warmed and viable for life, and so provides the primary energy needed to sustain life. Without the Sun, the Earth's atmospheric temperature would drop, and the planet's core would slowly cool, making it impossible for life to exist.
Therefore, the second statement is correct in that nuclear fusion in the form of the Sun provides both light and heat to the world, thus sustaining life and acting as the most important, or number one, source of energy.