2781. What characters can be added here so that each character contains a part of the previous character? a) 雷 -> _ -> _ -> 界 b) 架 -> _ -> _ -> 贈

a) 雷 -> 累 -> 細 -> 界  

雷  RAI, kaminari - thunder

累  RUI - involvement, trouble, accumulation

細  SAI, hoso(i), hoso(ru), koma(kai) - thin, narrow, slender

界  KAI - world

b) 架 -> 柱 -> 責 -> 贈  

架  KA, ka(keru), ka(karu) - hang, build (a bridge)

柱  CHUU, hashira - pillar, column, pole

責  SEKI, se(meru) - condemn, censure, torture

贈  ZOU, SOU, oku(ru) - give, present, bestow