A Waitress Is Interested in Whether She Receives Larger Tips

A waitress is interested in whether she receives larger tips in general from her male customers than her female customers. She records the gender of the customer who paid the bill and the amount of the tip over a two-day period. Statistically speaking, the "signal" in this research scenario would be

A) the difference between the average tip amounts received on the first and second days.
B) the fluctuation in tip amounts received from male customers during the two-day period.
C) the difference between the average tip amounts received from the male and female customers.
D) the fluctuation in tip amounts received from the male and female customers.