The explanatory variable in this case is "time spent streaming videos," and the response variable is "time spent exercising."
EXPLANATION
The explanatory variable is the variable that is presumed to affect or explain changes in the response variable. In this scenario, the researchers are examining how one variable, the time spent streaming videos, influences another variable, the time spent exercising. They are interested in understanding how changes in the amount of time spent watching videos might impact the time spent exercising among the teens they are studying. Therefore, the time spent streaming videos is the explanatory variable, while the time spent exercising is the response variable that is expected to change in response to variations in the time spent streaming videos.