An experiment is proposed to measure the effectiveness of a brand of sunscreen. Adult volunteers are randomly assigned to one of two brands of sunscreen and asked to enjoy a normal day at the beach. Each volunteer is asked to report back to the administrators after two hours, at which time both arms are assessed for sun exposure. Which statement describing this experiment is true?

The volunteers are the explanatory variables.
The brands of sunscreen are the response variables.
The amount of time in the sun is the response variable.
The brands of sunscreen are the explanatory variables.