Using the Underground Railroad slaves were not truly free until they
reached *

Until they reach the northern states

Explanation:

Technically even in the northern states, they were still kidnapped and moved to the south, so Canada is what most slaves wanted, but they were truly free when they reached a northern state.

answer; researchers typically cannot control people's sexual behaviors as would be necessary in an experiment;

the boundarys like convergent divergent and transform it causes subduction