[单选题]In the early 1850s, with the westward movement, the slavery issue became a serious political issue endangering the unity of the country because ___.
A
whether the future states formed as aresult of the west ward movement should be free or slave world affect the balance of power in the Senate
B
the southin sisted that slavery should beallowed tospread into all new territories
C
the north refused to let slavery spread into new territories
D
the north want edtoput and end to slavery
