Should an Observer Conceal or Disclose the Fact That Observations

Should an observer conceal or disclose the fact that observations are being made?

A) Concealed observation is preferred because it is less reactive.
B) Non-concealed observation is preferred because it is less reactive.
C) Concealed observation is preferred because the observer can take more precise measurements.
D) Whether concealed or non-concealed observation is preferred must be decided on an individual basis.