Answer:
B. demand for Apple computers decreases.
Step-by-step explanation:
If two things are substitute of each and other, that generally in simple words means either this or this. As we can see in the statements that has been provided in the question that Dell and Apple computers are substitute of each other, so if the amount from Dell computer decreases then the demand of the computers made by Apple company will absolutely decrease.
Because two devices which carries almost same qualities and features are also substitute of each other, if price of one device from the both substitutes will decrease, everyone will rush to buy the device with low price and the device with high price will get less popular among the consumers. So, this is the reason which says OPTION(B) is the correct answer.