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.

The explanatory variable is the factor used to explain or predict a change in another variable. Here, the number of text messages sent is associated with the percentage of phone memory used, so text messages are the explanatory variable and memory use is the response variable. Choice C names these variables correctly.

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