One use to which a gravity model can be put is as an assessment tool of the effectiveness of a trade agreement.

The figures above summarize data for a group of hypothetical countries. According to the figure on the$$ left, trade prior to any agreement among the countries is

$$

with predictions from the gravity model.

Part $3$

Suppose that countries D and B reach a trade pact that substantially lowers trade impediments between them. According to the figure on the$$ right, the agreement appears to have been

A$.$

helpful to A and C$.$

B$.$

effective.

C$.$

ineffective.

D$.$

No clear assessment is possible.