Samuel was performing a three-month study in which an experimental group received a new drug that was being tested, while a control group received a placebo. Toward the end of the first month, the control group was accidentally administered the new drug instead of the placebo. This accidental administration of the experimental group's drug to the control group is an example of which threat to external validity? 

a. Interaction between selection bias and the intervention 

b. Interaction effect of testing 

c. Multiple-treatment interference 

d. Reactive effect of the experimental arrangements