One of the Major Limitations of Behavioural Observations Is That

One of the major limitations of behavioural observations is that:

A) researchers know that people may behave differently when they know they are being watched.
B) researchers do not know what a given physiological response really means.
C) researchers do not know whether people's behaviours are internally valid.
D) researchers know that people may not behave differently when they know they are being watched.