When Avery's income is $100 per week, he spends $20 per week on sushi. When his income rises to $110 per week, he spends $25 per week on sushi. Assuming the price of sushi remains constant during this period, for Avery: a. sushi is a normal good. b. sushi is an inferior good. c. demand for sushi is price-elastic. d. sushi is a substitute good