Below Is a List of Measures of the French Influence

Below is a list of measures of the French influence in New Orleans. Which of the measures is NOT an unobtrusive measure?

A) the wear on novels in the New Orleans Public Library written in French
B) a survey using a three-page questionnaire partly written in French that was distributed to residents of a neighborhood
C) walking down a street in New Orleans and noticing that most of the signs in stores in a neighborhood are in French or French-Cajun
D) a box of 300 letters written by people living in New Orleans to relatives living in French speaking areas outside the state e.g., Quebec) between 1980 and 1985