Routinely in nature during a 24 hour period in nature, what substance do green plants continuously require and use?

The correct answer is **B. Carbon dioxide, only**.

During the day, green plants perform photosynthesis, which requires carbon dioxide (CO2) and sunlight to produce glucose and oxygen. However, in this context, the focus is on the continuous requirement, and during photosynthesis, carbon dioxide is used continuously by plants. At night, plants can also respire, which uses oxygen but does not involve carbon dioxide consumption. Thus, in terms of a continuous requirement throughout a 24-hour period, carbon dioxide is the key substance.