2697. What characters can be added here so that each character contains a part of the previous character? a) 粒 -> _ -> _ -> 黒 b) 章 -> _ -> _ -> 念

a) 粒 -> 音 -> 量 -> 黒  

粒  RYUU, tsubu - a grain

音  ON, IN, oto, ne - sound

量  RYOU, haka(ru) - quantity, measure, weigh

黒  The "Kuro" radical - black

b) 章 -> 古 -> 吟 -> 念  

章  SHOU - chapter, badge, mark

古  KO, furu(i) - old

吟  GIN - sing, chant, recite

念  NEN - thought, concern