A researcher uses a within-subjects design in which one group completes a difficult puzzle and the second group completes an easy puzzle. He finds that participants who completed the difficult puzzle first were able to learn about the puzzle and completed the easy puzzle quicker, compared to participants who did the easy puzzle first. What type of threat to internal validity was described in this example

experiments.

surveys.

questionnaires.

interviews.

case studies.

participant and non-participant observation.

observational trials.

studies using the delphi method.

earlier philosophers whose work influenced the enlightenment included bacon, descartes, locke, and spinoza. the major figures of the enlightenment included beccaria, diderot, hume, kant, montesquieu, rousseau, adam smith, and voltaire. it spread the ideas of the enlightenment across europe and beyond.