Which Is NOT an Example of an Observational Study

Which is NOT an example of an observational study?

A) Researchers assign participants to teach either older or younger people how to use a computer to test whether participants appear more frustrated teaching older people.
B) A manager uses sales records to compare the performance of different employees.
C) A professor studies the relationship between democracy and per capita income using country-level data from the World Bank.
D) A political scientist uses data from a random sample survey of 2000 US adults to examine the effect of media exposure on people's trust of government.