A. Gross Domestic Product per capita.
<span>C. the study of human behavior and mental processes</span>
The only answer related to the subject is "the results of their experiments".
After Scientific revolution scientists came up with theoretical conceptions and tried to prove them with experiments. There had to be both theoretical explanation and emirical proof. Before they were much more liberal when it comes to coming up with "scientific theorems", which explained results of their experiments.
