Which argument was used by southern slaveholders to justify slave labor?

It was critical to maintain the economy of the South.

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radioactive dating, also known as  radioisotope dating  is a process that in  estimating the approximate  age  of  rocks. these data can be analyzed from  the studies dealing with the  decay of the radioactive elements that are present in these rocks. in nature, there are a few types of materials or atoms which are quite unstable and they spontaneously decay or breakdown to form other kinds of atoms.

the answer for 3 is b and the answer for 4 is d