Which of the Following Is a Reason Why a Researcher

Which of the following is a reason why a researcher may design an experiment with more than two levels of an independent variable?

A) A design with only two levels of an independent variable cannot provide much information about the exact form of the relationship between the independent and dependent variables.
B) When there are only two levels of an independent variable, the relationship between the independent and dependent variables cannot be described with a straight line.
C) An experiment with only two levels of one independent variable always tends to show that there is a positive relationship between the independent and dependent variables.
D) An experimental design with only two levels of the independent variable can only detect curvilinear relationships between the independent and dependent variables.