(2) Photosynthesis Explanation: Plants reduce the CO2, because they use it with photosynthesis. They use it to give oxygen and carbohydrates as the final product. It is for this reason that plantation of trees has been stressed by environmentalists. 656. (4) Vitamin D is not really a vitamin, but a precursor for the most potent steroid hormone in the human body. It can be obtained from exposure to the sun's ultraviolet (UV) radiation, especially during the early morning hours.