Final answer:
In experiments, the independent variable is the condition or factor that is manipulated by the researcher to study its effect on the dependent variable. In the given example, 'the doughnuts' would be the independent variable. The correct answer for each study can vary depending on the details of what the researchers are manipulating or measuring.
Step-by-step explanation:
In Dr. Kang's study, determining which factor is the independent variable depends on what is being manipulated or controlled by the researcher. The independent variables in experiments are conditions or influences that are changed by the scientist to determine the effects on another variable, which is the dependent variable. For example, in a study where ten-year-old boys are fed doughnuts every morning for a week and then weighed to see how much weight they gained, the independent variable would be 'the doughnuts' because that's what the experiment changes to observe the effect on the boys' weight. Similarly, if we were analyzing the variables in Dr. Kang's study, options such as the length of the distractor task or the emotional or neutral content of the words could potentially be independent variables if they are factors that Dr. Kang is directly manipulating in the study.
In questions like the one about elderly drivers or the one concerning the number of letters in a state name depending on the year it entered the Union, the independent variable is something that is being used to compare against an observed outcome, such as the number of motor vehicle fatalities or the number of letters in the state name. Hence, the correct answer varies depending on the controlled factor in each specific study question.