Collective Behaviour Has Not Been the Topic of Much Empirical

Collective behaviour has not been the topic of much empirical investigation by social psychologists. Which of the following is a reason for the lack of social psychological data on this topic?

A) it is not really a social psychological phenomenon
B) there are ethical problems in studying collective behaviour in a laboratory setting
C) it is difficult to implement laboratory studies of collective behaviour
D) it is difficult to isolate and measure the relevant variables in field studies of collective behaviour
E) b c and d