Which action would be most important to perform in a controlled experiment?

• A. Change multiple variables at the same time
• B. Ignore sources of experimental error
• C. Keep all variables except the independent variable constant
• D. Record only the expected results

Answer: (C)

Verified step by step guidance

1. Understand the purpose of a controlled experiment: it is designed to test the effect of one independent variable on a dependent variable while keeping all other variables constant.
2. Recognize that changing multiple variables at the same time makes it impossible to determine which variable caused any observed effect, so this is not a good practice.
3. Identify that ignoring sources of experimental error can lead to unreliable results, so it is important to minimize and account for errors rather than ignore them.
4. Note that recording only the expected results introduces bias and reduces the reliability of the experiment; all data should be recorded objectively.
5. Therefore, the most important action in a controlled experiment is to keep all variables except the independent variable constant to ensure that any changes in the dependent variable are due to the independent variable alone.