问题如下：
In a market of perfect competition, if economic profits are accessible to companies. In the long run, the supply curve is most likely to:
选项：
A.stay the same.
B.shift to the right.
C.shift to the left.
解释：
B is correct
New competitors will be attracted to the market by economic profits, and the existing companies will be encouraged to expand production.
考点：完全竞争市场
解析：由于存在不菲的利润，所以这些利润会吸引外部的厂商进入该市场分一杯羹，同时原有厂商也会进一步扩大生产，两者都会导致行业供给增加，供给曲线向右平移。
想问perfect competition的供给曲线是什么样的，一条横线的是需求曲线，一条竖线是供给曲线？
