All of the following items were sources of wealth in Africa for European countries, except 1. Rubber
2. Slaves
3. Gold
4. Diamonds
5. Ivory
6. Coal
???

the answer is a. the declaration of independence.

explanation:

the addition to geographic boundaries that might have  kept the people of the huang river civilization from traveling to far distance region is that they had learned about the other river valley civilizations but feared contact would bring invasion.

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most people now work in jobs where they do one  kind  of work; they are carpenters, bankers, cooks, mechanics, and so forth. likewise, most  businesses produce  only certainkinds  of  goods  or  services, such as cars, tacos, or gardening  services. this feature of  production  is known as specialization.