Answer:
The correct answer is: 3. nothing to compare the results to.
Step-by-step explanation:
When conducting a scientific experiment, including a control variable is essential to establish a baseline or reference point for comparison. The control variable serves as a standard against which the results of the experimental variable are measured. Without a control variable, there is no basis for comparison, making it difficult to determine the effectiveness or impact of the experimental variable. This absence of a comparison undermines the validity and reliability of the experiment, resulting in poor scientific practice.