Why Is It Important for Researchers to Use a Control

Why is it important for researchers to use a control group in a scientific study?

A) The control group receives the experimental manipulation or intervention and is therefore the reason one would conduct a scientific study in the first place.
B) When a researcher manipulates the control group, it is always under carefully scrutinized and supervised conditions, thereby increasing the validity of the study.
C) Only when participants listen carefully and follow precisely the instructions of the experimenter will the study be controlled and thus yield valid scientific conclusions.
D) It is important to show that between two groups which were similar from the beginning of the study, theone that received the manipulation is the one that changed.