Indicate whether each of the following studies is an experiment or an observational study. If it is an experiment, identify the independent variable and note any possible confounding variables.

A. A psychologist uses chimpanzees to test the notion that more crowded living con-ditions trigger aggressive behavior. Chimps are placed, according to an impartial assignment rule, in cages with either one, several, or many other chimps. Subse-quently, during a standard observation period, each chimp is assigned a score based on its aggressive behavior toward a chimplike stuffed doll