Which of the Following Are Not Illustrative of Unobtrusive Observations

Which of the following are not illustrative of unobtrusive observations?

A) Examining the floor tiles at a museum to determine which exhibits are the most popular
B) Examining the number of beer cans in the university garbage collections to determine beer consumption patterns
C) Examining the wear on the tires of squad cars to determine the extent of police patrols
D) Examining the radio dial settings of cars brought in for oil changes to determine the popularity of radio stations
E) Examining domestic violence by conducting interviews