【单选题】Tammy buys four Barbie dolls a year if the price of a doll is $12 per doll. The actual price of a Barbie doll is $13 per doll. The four dolls Tammy buys if the price is $12 are her
A.
demand for Barbie dolls.
B.
quantity demanded of Barbie dolls when the price is $12 per doll.
C.
quantity demanded of Barbie dolls when the price is $13 per doll because the price is $13 per doll.