问题如下：
What is the key driver of trade in Ricardian trade model?
选项：
A.Technology.
B.The level of labor productivity.
C.Relative amount of labor and capital.
解释：
A is correct.
In the Ricardian model, differences in technology are the source of comparative advantage and hence the key driver of trade.
考点：Ricardian mode
Ricardian mode认为技术进步以及劳动生产率是比较优势的来源，所以是国际贸易中至关重要的因素。
老师，有tecnologies和labor productivities的时候，不是优先选择后者么
