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.