Which of the following describes the swahili civilizations relationship with the people who lived in the interior?
a. swahili kings expanded in the interior eventually establishing an empire that stretched from coast to coast
b. swahili states demanded that people living in the interior lay an annual tribute to gain access to the coastal trade.
c. swahili cities operated intermediaries for people from the interior sell their good to arab merchants
d. swahili cultures spread throughout most of the interior as evidence by widespread acceptance of islam