Why Do Researchers Change Only One Variable at a Time

Multiple Choice

Why do researchers change only one variable at a time in single-case experiments?

A) There are not enough participants to change multiple variables at once.
B) There is increased external validity when only one variable is changed at a time.
C) Changing multiple variables at once creates confounds in the experiment.
D) The statistical techniques required to analyze the data are very complex when more than one variable is changed at a time.