OPSC 2015 Solved Paper

Most plants carry out photosynthesis during the day, absorbing carbon dioxide and release oxygen; night respiration, absorbing oxygen and release carbon dioxide. And some plants, by contrast, such as the cactus is the release of carbon dioxide during the day and night then absorb carbon dioxide and release oxygen, so that put a cactus in the room at night, you can add oxygen and conducive to sleep.