Final answer:
A control group is important as it provides a point of comparison and ensures that observed effects in the experiment can be attributed to the experimental manipulation by keeping other variables constant.
Step-by-step explanation:
A control group is crucial in a scientific experiment because it serves as a basis for comparison. It allows the researcher to control for variables that might otherwise influence the results of the study by keeping such factors constant. This ensures that any observed effects can be attributed to the experimental manipulation rather than other variables. For instance, in a study on the effect of a nutrient on algae growth, the control group would receive a placebo instead of the actual nutrient. This helps in determining if the nutrient truly has an effect on algae growth by comparing the results against the control group which did not receive the nutrient.
It is important to only change one variable at a time in a scientific experiment to ensure that any observed changes in the dependent variable are due to the manipulation of the independent variable. This is essential for establishing a cause-and-effect relationship. Therefore, the correct answer to why a control group is important is that it is a comparison to the group where variables are changing, helping to isolate the effects of the experimental variable.