What were colonial powers interested in mining what resource that was abundant in africa?

Which would make the best heading for this list of notes about the migration of african-americans during the reconstruction era?

Which is one reason many african nations have not prospered after gaining independence from european colonial powers? a. colonial powers sent their weakest economic advisors to the new countries, and their recommendations were flawed. b. independence left them with weak economies, and many countries have been unable to overcome or recover from these burdens. c. the desert region of the interior is inhospitable to foreign investors, and business interests focused on the coastal mediterranean cities. d. the money endowed upon independence is still tied up in the bank of england, which is hesitant to make loans to struggling nations.

Why did the french settle in new france