Legally Speaking, the Geographic Concentration of Slavery in the Southern

Legally speaking, the geographic concentration of slavery in the southern part of the U.S. is explained by all of the following except

A) provisions in the Northwest Land Ordinance of 1787.
B) provisions in the constitutions of some northern states.
C) laws allowing for gradual emancipation in some northern states.
D) an amendment to the U.S. Constitution that allowed importation of slaves only through the port of Charleston after 1800.