Indicate whether each of the following constitutes data collected from a population or a sample. a. Salaries

Question:

Indicate whether each of the following constitutes data collected from a population or a sample.
a. Salaries of CEOs of all companies in New York City
b. Five hundred houses selected from a city
c. Gross sales for 2012 of four fast-food chains
d. Annual incomes of all 33 employees of a restaurant