Naturalistic observation watches behaviour where it normally happens; structured observation stages the situation in a controlled setting. Why does gaining control cost you something that cannot simply be added back?

D. Because control is achieved by removing the ordinary context.

Control is obtained by stripping out variation, and the stripped-out variation is the ordinary context, so if playground aggression depends on crowding and escape routes, the playroom has removed the things doing the work. The strongest evidence is finding the same result in both, since the two methods' weaknesses are different and no single alternative explanation covers both.