Ascientist grows tomatoes in greenhouses that differ only in the amount of carbon dioxide in the air. he determines the weight of tomatoes produced by plants in each greenhouse, and compares them to each other and to tomatoes grown in air with a natural, unaltered amount of carbon dioxide. is this a controlled experiment? why or why not?

Yes, this is a controlled experiment.

Explanation:

The scientist grows tomatoes inside the greenhouse with altered amount of carbon dioxide. So he is changing an important factor required for the healthy growth of plant and increased growth of fruits. So this is a controlled experiment.

But the size of tomatoes depends not only on one factor i.e. Carbon dioxide, but also depends on the other factors like sunlight, amount of water, temperature and the saturation of the photosynthesizing capacity of that particular plant.