The classical experimental design: a. helps guard against the sources of internal invalidity. b. and what it helps guard against depends upon the particular experiment. c. cannot guard against the sources of either internal or external invalidity. d. helps guard against the sources of external invalidity. e. helps guard against the sources of internal and external validity.

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.