7. why did the southern colonies switch to using slaves instead of indentured servants

They were considered to be cheap source of labor when compared to indentured servants and African slaves were in use for some thousands of years in American colonies.  

Indentured servants were mostly from Virginia and they were rebellious. They possessed weapons and arms in order to protect themselves from pirated and burglars. African slaves were not permitted to be equipped with weapons. They were made slaves on the basis of their color and ethnicity.

Slaves included both men and women who were capable of doing farm work but indentured were only men and their rebellious attitude made the English men to prefer slaves than indentured servants.

a historian researching the immigration policies of the early 1900s would expect a source written today,rather than in 1910 to :

( a ) be more interested in objective data and statistics.