C. Like question 27, this question evaluates your ability to read and understand data in graphical form. According to the figure, at light intensities between 100 and 150 ?mol photons/m2/sec, CO2 assimilation in shade leaves is greater than that in sun leaves. Two hypotheses are plausible: shade leaves are assimilatingmore CO2 than sun leaves because they are doing more photosynthesis than sun leaves or shade leaves are releasing less CO2 because they are doing less cellular respiration than sun leaves. This latter hypothesis occurs because under low light conditions, some cells may be doing photosynthesis while other cells may be doing cellular respiration. The hypothesis in answer choice C describes this scenario where the shade leaves are releasing less CO2 because more cells are photosynthesizing than respiring compared to sun leaves. For these leaves, apparently, shade leaves are more efficient than sun leaves under low light conditions. Answer choice A is not supported by the data because the CO2 assimilation data in the figure is independent of leaf surface area (CO2 assimilation is per m2). Answer choice B is incorrect because respiration releases CO2, the opposite of CO2 assimilation. Answer choice D contradicts the data presented in the figure, as shade leaves are more efficient than sun leaves for light intensities between 100 and 150 ?mol photons/m2/sec. Therefore, only answer choice C supports one of the two plausible hypotheses that could explain the data in the figure.