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.