From 2010 to 2015, a group of researchers conducted an experiment using two types of pain medication: pill A and pill B. The researchers gave the doctors the pills in unidentified packages so they would not know which package was pill A and which package was pill B. The patients participating did not know which pill they received either. What is this an example of?

a Group of answer choices

b The effect of a treatment unit

c The placebo effect

d A control group study

e A double-blind study

f Voluntary response bias