2749. What characters can be added here so that each character contains a part of the previous character? a) 炉 -> _ -> _ -> 軟 b) 弦 -> _ -> _ -> 縛

a) 炉 -> 扉 -> 輩 -> 軟  

炉  RO - furnace, hearth

扉  HI, tobira - door, title page

輩  HAI - fellow, colleague, companion

軟  NAN, yawa(rakai), yawa(raka) - soft

b) 弦 -> 畜 -> 累 -> 縛  

弦  GEN, tsuru - string, bowstring

畜  CHIKU - animal raising, domestic animal

累  RUI - involvement, trouble, accumulation

縛  BAKU, shiba(ru) - tie up, bind