Read the passage below and choose the correct answer for questions 126-130

The sun is the most direct source of energy. It powers the flow of wind and water cycle and sustains life. Plants use this energy to synthesize carbohydrates from simple substances like carbon dioxide and water. All the food is derived from the process of photosynthesis. In fact, energy by which all animals including human beings live is generated by the oxidation of the food produced by the plants. The sun contains in its core hydrogen nuclei moving at very great speeds. Whenever these nuclei collide and fuse to form a nucleus of heavier element it results in nuclear reactions. These reactions generate tremendous amount of energy. It is this energy that power sun.

The sum emits lights of different wavelengths. If sunlight is passed through a prism each of these wavelengths is refracted by a different amount. Violet has the shortest wavelength and red has the longest. The wavelength of green is midway between that of violet and red. Light with wavelength shorter than that of violet is called ultraviolet light. Light with wavelength longer than that of red light is called infrared light. About one third of the light from the sun is infrared. We know that nuclear reactions that go on in the interior of the sun liberate a large amount of energy. Nuclei of deuterium, which is the heavier isotope of hydrogen, collide in the sun's interior to produce helium. The energy liberated in these reaction fires the sun, which, in turn, emits lights of different wavelengths. Of these wavelengths, it is the infrared wavelengths that heat up the earth. The reaction in which the hydrogen in the sun is converted into helium is called a fusion reaction.