Final answer:
Sulfur oxides emitted by plants primarily originate from the combustion of sulfur-bearing fuels like coal, as well as from electric utilities, industrial processes, and natural volcanic eruptions.
Step-by-step explanation:
Many plants emit sulfur oxides, which primarily originate from the combustion of sulfur-bearing fuels. These fuels include coal, which often contains a significant amount of sulfur. During the combustion process, particularly at high temperatures, sulfur within the fuel reacts with oxygen to form sulfur dioxide and sulfur trioxide. These sulfur oxides can lead to environmental concerns such as acid rain and smog. Additionally, other anthropogenic sources like electric utilities and various industrial processes contribute to the emission of sulfur oxides. Meanwhile, natural sources such as volcanic eruptions also release massive amounts of sulfur dioxide into the atmosphere.