Which of the Following Is a Drawback to the Observational

Which of the following is a drawback to the observational method?

A) People's behaviours often change when they know they are being observed.
B) The observational method requires researchers to interact with their subjects for long periods of time.
C) No interesting or important questions can be answered using this method.
D) The observational method is statistically unreliable.
E) The observational method cannot impact a well-accepted theory.