What is the correct answer? (AP statistics)

In a famous study from the late 1920s, the Western Electric Company wanted to study the effect of lighting on productivity in a factory. The factory workers were aware that the study was in progress. The researchers discovered that worker productivity increased with each change of lighting, whether the lighting increased or decreased. What's the most likely reason this phenomenon occurred? Lack of randomization Absence of a control group The subjects knew they were being studied Sampling variability overpowered other kinds of variability Voluntary response bias and undercoverage