How did westward expansion contribute to the secession of southern states, including south carolina? a) southerners feared that the addition of free states would allow the north to gain more power in congress. b) southerners worried that the constitution did not allow for the additions of "slave" states to the union c) southerners argued that a free state should not be added without the addition of a slave state at the same time. d) southerners did not support westward expansion and argued fiercely against territorial overreach,