A researcher wants to test the hypothesis that the number of bystanders affects the probability of helping behavior. She sets up three conditions, one with no bystanders, one with two bystanders and one with four bystanders. Which of the following is true?

A. there is one independent variable having three levels
B. there are three independent variables having one level each
C. there is one independent variable and three dependent variables
D. there is one independent variable having four levels