Answer:
B (They had to export goods that they had in surplus in order to import scarce resources.)
Step-by-step explanation:
Think about the US today. There are some resources - oil, for example - that we trade for because we do not have enough of them to sustain ourselves. In return, we pay money that the other countries can use to help the people. The Babylonians did not have enough - metal, for example - to sustain themselves, so they bartered with others (not using money, of course). Therefore, to pay for the resources they did need, they had to have a surplus of other natural resources to trade. They exported these goods to others and imported resources that were scarce.
I wish you the best of luck ;)