Carbon fixation is the process in which plants remove the carbon from atmospheric carbon dioxide and turn it into organic molecules like carbohydrates.
Because different type of plants are located in regions with different conditions they are different types of carbon fixation. Plants that live in arid regions need to conserve water, while plants that live in more moist conditions will not need to conserve water.
The carbon fixation in C3 plants is one-step process. An oxidation reaction occurs because of the enzyme RuBisCo. During the oxidation some of the energy used in photosynthesis is lost in a process known as photorespiration.
If CO2 was in very small amounts then it would be the limiting factor of photosynthesis, this means the process will take place at a much slower rate because it is lacking one of the raw materials it needs for the process to occur. To find the optimum light intensity you really need all other factors to be at optimum levels or in abundance.
B is the correct answer. They can sometimes show an extinction but not always.
Because bacteria can be good for the environment