645. What characters can be added here so that each character contains a part of the previous character? 粒 -> _ -> _ -> 胞

Answer:

粒 -> 音 -> 明 -> 胞

粒  RYUU, tsubu - a grain

音  ON, IN, oto, ne - sound

明  MEI, MYOU, aka(rui), a(keru) - bright

胞  HOU - sac, sheath, placenta