A Researcher Is Studying the Effect of Including a Bonus

A researcher is studying the effect of including a bonus product on people's tendency to purchase personal care items (e.g., shampoo, deodorant, etc.) . The researcher randomly assigns male and female consumers to be asked to buy the product with the bonus or to be asked to buy the product without the bonus. The researcher then calculates the proportion of people in each condition that purchase the product. The independent variable in this study is:

A) the proportion of people that purchase the product.
B) personal care items.
C) gender.
D) the presence or absence of a bonus.