(1) Plants will absorb T- carbon oxides by photosynthesis under light, and photosynthesis needs light; The black plastic bag is opaque to prevent the plants in the plastic bag from affecting the experimental results due to photosynthesis.

(2) Tertiary carbon oxides will make clear limewater turbid, and plants will produce U amount of tertiary carbon oxides through respiration. After a period of experiments, fresh vegetables can breathe, but scalded vegetables can't. Therefore, it is found that bag A can make the lime water in the conical bottle turbid, but bag B can't. According to the experiment, it can be judged that bag A contains fresh vegetables.

(3) Mitochondria are places for breathing. Oxygen is used on mitochondria in cells to decompose organic matter into carbon T oxides and water, and the energy stored in organic matter is released to meet the needs of life activities. This research experiment proves that living cells of plants can breathe and produce carbon dioxide.

So the answer is:

(1) Photosynthesis?

(2) Answer

(3) Inhale carbon dioxide