Cell phone production engineers have noticed that among a group of 138 customers aged 14-23 who send more text messages tend to use a higher percentage of their phone’s memory.What are the explanatory variable and response variable for this relationship?

Explanatory variable: age of customer
Response variable: percentage of phone memory used
Explanatory variable: percentage of phone memory used
Response variable: age of customer
Explanatory variable: number of text messages sent
Response variable: percentage of phone memory used
Explanatory variable: percentage of phone memory used
Response variable: number