Which experiment method is important for controlling bias ?

Which is the best conclusion that can be drawn regarding the status of african-americans in the south in the late 1800s? a) african-americans made tremendous political, social, and economic advancements during the reconstruction era. b) passage of the 13th, 14th, and 15th amendments to the u.s. constitution guaranteed equal status and rights for all african-americans. c) the reconstruction policies enacted by the federal government were highly effective in african-american citizens transition to life after slavery. d) although african-americans had equal rights under federal law they were still economically, politically, and socially inferior to whites in the south during the reconstruction era.