BII Reading 对应几年级Kumon workbook？

My son is studing Reading and Math in Kumon Center.  I want to let him to do Reading (Level BII) at home because Center forces him to repeat too much.  When I browse Kumon workbook in Amazon, workbooks are catagized by Grade 1,2,3.. But Kumon Center uses Level BII, CI, CII...  How do I match two systems?
Thanks