Researchers use experiments rather than other research methods in order to distinguish between

causes and effects

Explanation:

Through experiments, cause-and-effect can be easily understood by practically showing what outcome occurs (effect)  when a particular independent variable is manipulated (cause). Researchers prefer using experiments in establishing or ascertaining the cause-and-effect relationship between two variables under study. It also enables researchers to manipulate  one variable in order to determine the effect it would have on the other variable. In this type of research method, we have the independent variable and the dependent variable. The independent variable is the “cause” that is often manipulated, while the dependent variable is the “effect” resulting from the manipulation of the independent variable.

the answer is "a.) well–developed vascular system."

explanation:

a physical change is being made.