These people became wealthy mostly because of efficient trade between countries. Trading was an important factor back then, and your key word is COASTAL, meaning they are by a sea (aka trading routes). So the answer is trading between countries.
For the answer to the question above, I believe that the
<span>international trade routes led to contact with <u><em>India and China</em></u>
</span>I hope this helped you. Have a nice day!
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