A, before the light spot starts, the amount of oxygen released is equal to the amount of carbon dioxide absorbed, indicating that the light reaction rate of photosynthesis is equal to the dark reaction rate, so A is wrong;

B, after the "spot" irradiation began, the release rate of O2 increased sharply, and the absorption rate of CO2 was relatively slow, indicating that the light reaction and dark reaction did not increase synchronously, so B was wrong;

C, the release rate of O2 decreases obviously, which is equivalent to the absorption rate of CO2, indicating that the dark reaction limits the light reaction, so C is correct;

D, AB segment "light spot" moved away, the light weakened, the bright reaction weakened, and finally the dark reaction gradually weakened, so D was wrong.

So choose: C.