Experiments:

A$.$ make it easier to determine whether people's choices are consistent with standard economic theory, but can make it harder to establish causality

B$.$ make it harder to establish causality and to determine whether people's choices are consistent with standard economic theory

C$.$ often make it easier to establish causality, but can make it harder to determine whether people's choices are consistent with standard economic theory

D$.$ make it easier both to establish causality and to determine whether people's choices are consistent with standard economic theory