Carrot cells need carriers and energy to absorb K+ and NO3- through active transport (energy is provided by aerobic respiration, which is closely related to oxygen concentration, and oxygen concentration in a certain range represents the amount of energy), so the number and energy of carriers are the main factors affecting the absorption of these two substances.

The two points A and B are on the same curve, and the absorbed ions are the same, so the carriers are the same, so the factor that affects the different absorption capacities of the two points A and B is energy. The two points B and C are not on the same curve and absorb different ions, but the oxygen concentration at the two points is the same, which means the energy is the same, so the factor that affects the absorption capacity of the two points B and C is the number of carriers.

So choose B.