In Order for Peking Ducklings to Develop an Appropriate Following

In order for Peking ducklings to develop an appropriate following response, Gottlieb and coworkers demonstrated that they must

A) be reared in isolation so that they are not exposed to conflicting stimuli.
B) hear calls typical of another species, such as a chicken, to acquire appropriate discriminations.
C) hear their own or their siblings' contentment calls after hatching.
D) hear their own or their siblings' contentment calls before hatching.