Answer:
The correct answer is a.
Explanation:
Internal validity refers to the establishment of a proper cause and effect relationship in an experiment. Internal invalidity is the opposite, the possibility that the conclusions from the experiment aren't actually demonstrating a proper cause and effect relationship. The classical experiment design helps guard against the sources of internal invalidity through three main elements that must be present: testing how an independent variable affects a dependent variable; measuring the pre-testing and post-testing results of the experiment; and having an experimental and a control group, in order the compare the differences when the variables change.