Which of the following is an example of the use of a secondary data source

a: a researcher assigns different foods to two treatment groups and records the cholesterol levels of different participants after 3 months
b: a researcher interviews people about their eating habits and compares their answers and cholesterol levels
c: a researcher watches people in the cafeteria and records their eating habits then asks them to have their cholesterol tested and report the results.
d: a researcher searches the internet and finds information that relates eating habits and cholesterol levels.