Final answer:
A decrease in the price of a substitute product can cause a rightward shift in the demand curve for product C.
Step-by-step explanation:
A rightward shift in the demand curve for product C might be caused by a decrease in the price of a product that is a close substitute for C. When the price of a substitute product decreases, consumers are more likely to choose the substitute over product C, resulting in a decrease in the demand for product C. This decrease in demand is represented by a rightward shift in the demand curve.