A scientist investigated the rate of photosynthesis of one type of tomato plant. The tomato plants were grown in a greenhouse. Table 2 shows the results.

Give two control variables the scientist should have used in the investigation.

In any scientific investigation, it is crucial to keep all variables constant except for the independent and dependent variables. This ensures that any observed change in the dependent variable is due to the change in the independent variable, making it a fair test. The rate of photosynthesis is affected by several limiting factors: - **Carbon dioxide concentration** (the independent variable in this experiment). - **Light intensity:** More light (up to a point) increases the rate. This must be kept the same for all plants. - **Temperature:** Photosynthesis is controlled by enzymes, which have an optimal temperature. The temperature must be kept constant. - **Amount of water:** Water is a reactant in photosynthesis. All plants should receive the same amount of water. Therefore, light intensity and temperature are two key control variables.