Consider the markets for mobile and landline telephone service. Suppose that when the average income of residents of Plainville is $55,000 per year, the quantity demanded of landline telephone service is 12,500 and the quantity demanded of mobile service is 28,000. Suppose that when the price of mobile service rises from $100 to $120 per month, the quantity demanded of landline service decreases to 11,000. Suppose also that when the average income increases to $60,000, the quantity demanded of mobile service increases to 33,000.