The best response is:
Northern states viewed enslaved people as property, thus they believed that enslaved people should not be counted as full citizens.
This reflects the perspective of many Northern states during the debates over representation and slavery in the Constitution, where they argued that enslaved people should not be counted as part of the population for political representation because they considered them property rather than citizens. Consequently, they were concerned about the political power the South would gain if enslaved individuals were counted for representation in Congress.