Why would someone argue that the north was complicit in the expansion of slavery?

Which of the following resulted from trade between nomadic people of central asia and civilizations of india during the vedic age? a.the people of the vedic age died from new diseasesb.the people of the vedic age could better obtain cattle and other goodsc.the nomadic people of central asia learned about shipbuildingd.the nomadic people of central asia wrote epic poetry