To determine which variable is being tested in this experiment, we need to analyze the experimental design outlined in the table. The table compares different factors between an experimental group and a control group. For each factor, we see whether it is the same or different between the two groups.
Let's look at the factors one by one:
1. Amount of water: Both the experimental group and the control group receive water daily. Therefore, this variable is not being tested because it is consistent across both groups.
2. Type of food: Both groups are given fresh grass as their type of food. Since there is no difference between the groups in terms of the type of food, this variable is also not being tested.
3. Housing: Both groups are housed indoors. Again, this variable is not being tested because it remains the same for both groups.
4. Vitamins: The experimental group receives vitamins (yes), while the control group does not receive vitamins (no). This is the only factor that differs between the two groups.
Since the only difference between the experimental group and the control group is the presence of vitamins, the variable being tested in this experiment is the vitamins.
Therefore, the answer is:
D. Vitamins
