In language, a conjunction is a word used to connect different clauses or sentences and that shows the relationship between the elements connected. In the case of Hava, as she wants to explain a reason or explanation she should select conjunction related to cause such as because, as, for the reason that, etc. Therefore, the word that completes the sentence "I visited Poland _____ my parents were born there" is because as this is the only conjunction that expresses the relationship of cause or explanation in the sentence.
The majority of them were positioned randomly in a field segregated from everything else. They were extremely lucrative and very self-sufficient considering that slave states had people working there for free.