Context: Proto-Slavic, the supposed ancestor language of all Slavic languages, is a descendant of common Proto-Indo-European, via a Balto-Slavic stage in which it developed numerous lexical and morphophonological isoglosses with the Baltic languages. In the framework of the Kurgan hypothesis, "the Indo-Europeans who remained after the migrations [from the steppe] became speakers of Balto-Slavic".

Question: What is the supposed ancestor of all Slavic languages?
Answer: Proto-Slavic

Question: Proto-Slavic developed numerous lexical and morphophonological isoglosses with what languages?
Answer: Baltic languages

Question: The Indo-Europeans who remained after the migrations became speakers of what?
Answer: Balto-Slavic

Question: Who became speakers of Balto-Slavic?
Answer: Indo-Europeans

Question: Proto-Slavic is a descendant of what?
Answer: Proto-Indo-European

Question: What does Proto-Indo-European descend from?
Answer: Unanswerable

Question: How did Proto-Slavic become the ancestor language to all Slavic?
Answer: Unanswerable

Question: What did Proto-Indo-European form with Balto languages?
Answer: Unanswerable

Question: What proposes the Indo-Europeans that migrated became speakers of Balto-Slavic?
Answer: Unanswerable