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. plantations employing high slave labor were present in Southern 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.

d. an amendment to the US Constitution that allowed importation of slaves only through the port of Charleston after 1800.