How often is halley's comet visible from earth? every 17 years every 100 years every 20-30 years every 75-76 years

Hurr. how did lincoln respond to the events at fort sumter? a)he called for the mobilization of troops to put an end to the rebellion. b)he signed an ordinance to dissolve the confederacy. c)he called for the seizure of federal property throughout the south. d)he declared the end of slavery in the united states.