According to the article, "Run Field Experiments to Make the Most of Your Big Data," by Zoumpoulis, Simester, and Evgeniou, what are cross$-$product effects?

When salesperson turnover affects multiple customers.

When changing the packaging of products increases costs.

When changing something about one product affects the way customers purchase another product.

When purchasing one product requires that customers also purchase another product.

None of the above.