A major increase in cigarette taxes occurs during a one-year follow-up study of the impact of a smoking cessation program. Identify the main threat to internal validity (History, Instrumentation, Maturation, Mortality, Selection Bias, or Testing).

The main threat to internal validity here is History.

Explanation:

Internal validity refers to how well a research is conducted. Internal validity determines if it's the independent variable (or another factor) that caused or resulted in the change in the dependent variable.

History here alludes to the internal validity threat that refers to events outside of the experimental setting (increase in cigarette taxes) that may affect the dependent variable (smoking cessation).

we need the diagram to answer this correctly.