When the price of a product changes, O A. it changes the relative price of the product causing a network effect and at the same time it changes the purchasing power of the buyer causing an income effect as well. O B. it only causes an income effect by changing the purchasing power of the consumer. O C. it only causes a substitution effect by changing the relative price of the product. D. it changes the relative price of the product causing a substitution effect and at the same time it changes the purchasing power of the buyer causing an income effect as well