Assume that the domestic demand for mp3 players is given by P = 80 - Q. The supply of domestic producers is given by P = 10 + Q, and the international supply by P = 15. If this is an open economy then the number of quantities of
import of mp3 for this economy is:

a)   50
b)  60 c)   40 d)   20


答案是选择B 为什么呢