Which of the Following Would Be an Example of Observer's

Which of the following would be an example of observer's paradox?

A) children under study stop talking about "boogers" when the principal walks in to the class
B) while having lunch with a research participant's home,she changes the topic of conversation when her husband walks in to the room
C) an hour into an interview,a research participant relaxes and starts talking much more informally,using some profanity and colloquialisms
D) while doing research,a researcher seeks to speak informally,adopting local idioms
E) during an interview,a research participant refrains from swearing