A survey records many variables of interest to the researchers conducting the survey. Below are some of the variables from a survey conducted by the U. S. Postal Service. Which of these variables is categorical?

(A) Number of people, both adults and children, living in the household.

(B) Age of respondent.

(C) Total household income, before taxes, last year.

(D) County of residence.

(E) None of the above.