Which of the following statements is the best example of a researcher making a causal inference?
1) researcher concludes that the health of unborn babies was negatively impacted by the high levels of stress their mothers experienced during pregnancy.
2) A researcher concludes that 85 percent of all CEOs in the United States have annual incomes of over $200,000.
3) A researcher concludes that the health of unborn babies co-varies with the amount of stress their mothers received during pregnancy.
4) A researcher concludes that 20 of the 35 people in her sample are suffering from depression.