The best explanation for the scientist's findings—that the amount of carbon dioxide has increased and the amount of oxygen has decreased—would be:
Only respiration occurred because the plant was in the dark.
In the absence of light, plants cannot perform photosynthesis, which uses carbon dioxide and produces oxygen. Instead, they would rely on cellular respiration, which consumes oxygen and releases carbon dioxide. Thus, the observed increase in carbon dioxide and decrease in oxygen can be attributed to respiration occurring in the plant in the dark.