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 of text messages sent