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 a result of the westward movement should be free or slave would affect the balance of power in the Senate
B. the south insisted that slavery should be allowed to spread into all new territories
C. the north refused to let slavery spread into new territories
D. the north wanted to put an end to slavery