To determine which variable is being tested in the given experiment, we need to compare the conditions of the experimental group with those of the control group. The variable that is different between the two groups is the one being tested. Let's go through the details step by step:
1. Days of growth: Both groups have the same "Days of growth" condition set to "1". Therefore, this variable is not being tested.
2. Temperature: Both the experimental and control groups are kept at the same "warm" temperature. Hence, "Temperature" is not the variable being tested.
3. Type of culture medium: Both groups use a "rich" type of culture medium, so this variable remains constant and is not being tested.
4. Antibiotic: This is where the difference lies. The experimental group is subjected to an "Antibiotic" (yes), whereas the control group is not (no). This means that the variable "Antibiotic" is the only factor that changes between these two groups.
Since the use of "Antibiotic" is the variable that is different between the experimental group and the control group, it is the one being tested.
Thus, the correct answer is:
C. Antibiotic
