Explain why, at point X, carbon dioxide is neither taken up nor given out by the tomato plants. (1)

This is an controlled experiment because he is only changing the amount of carbon dioxide in the air. Since he is only changing one variable at a time and comparing it to a natural, unaltered tomatoe area, his experiment is controlled.

Explanation:

The definition of a controlled experiment is a test where the person conducting the test only changes one variable at a time in order to isolate the results. An experiment where all subjects involved in the experiment are treated exactly the same except for one deviation is an example of a control experiment.

not sure if this is what you'r looking for eubacteria and archaebacteria

the titanic would be the correct answer