What advantage did the allies have at the outbreak of world war 1

How did european exploration and expansion cause the african slave trade to expand? a. european explorers discovered the slave trade and expanded it across the indian ocean into south asia. b. arab armies captured european explorers and forced them into slavery. c. european traders shipped enslaved africans to work on plantations in european colonies in the americas. d. european explorers captured african people and forced them to work on their ships.