What was the immediate cause of secession of southern states from the union prior to start of the civil war?

During andrew johnson’s presidency, the radical republicans gained control over policymaking in congress. they had enough power to override any presidential veto. how did the radical republicans use this opportunity? a. they blocked johnson’s weak reconstruction policies by taking charge of the reconstruction plan. b. they extended their support to johnson’s policies for reconstruction of the south. c. they removed johnson from office and took over the task of reconstructing the south. d. they removed all former confederates from office and stopped the south from rejoining the union.