Which observations would support the claim that as plants absorb sunlight to make food, they release oxygen into the environment?

A. Oxygen concentration in the atmosphere was much higher millions of years ago when different plants existed

B. When plants are kept in a sealed environment, the concentration of carbon dioxide steadily goes down

C. Plants that are kept under condition of total darkness fail to grow normally and eventually will die

D. The oxygen concentration in a pond with many plants increases during the day and decreases at night and on cloudy days