Three plants were grown to study the effects of nitrate and magnesium ion deficiency on their development. They were kept in the same conditions, except for the types of minerals supplied. Use
Plant A was provided with all essential minerals.
Plant B was given all minerals except nitrate ions. Plant C was given all minerals except magnesium ions. State three conditions, other than water and the concentration of mineral ions, that would need to be kept the same for all the plants, in order to make the investigation a fair test

Explanation:  

Controlled variable: Variables that are controlled and have no influence on the results of the experiment. These variables do not affect the change in the dependent variable values.  

These variables must be the same for all the subjects or groups (control and experimental). In the case of plants, controlled variables are probably those needed by the plants to correct development.  

All plants should be exposed to the same period of light-time and the same temperature. Also, the substrate or soil where the plants are growing should be the same.

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