What could be a reason for the southern economic worth being so much higher than that of the new england and the middle colonies?

The reason for the southern economic worth being so much higher was that they developed a plantation system. And the plantation system succeeded due to headright.
Headright means the land given to the settlers to grow crops and this became famous in colonial era where people were given from one acre to one thousand acres for plantation. This resulted in a booming system of plantation.

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answer; /// i believe that the correct answer is (d)the huang he river valley;