The glass bottle is filled with cold boiled water, and sodium bicarbonate (CO2 releasing agent) is added, so that the Ceratophyllum in the device A can carry out photosynthesis with carbon dioxide and water under the illumination condition, and release oxygen; Nails in A are more exposed to oxygen and moisture at the same time, and rust more easily than nails in B. 。
So, the answer is: a; Photosynthesis produces oxygen.