The Textbook States That Pavlov "Had to Pick a Stimulus

Multiple Choice

The textbook states that Pavlov "had to pick a stimulus that was more controlled than the sound of someone walking into a room" in order to investigate classical conditioning more systematically. In other words, Pavlov had to deliberately manipulate the neutral stimulus to conduct:

A) correlational studies.
B) case studies.
C) observational research.
D) experimental research.