2221. What characters can be added here so that each character contains a part of the previous character? a) 炎 -> _ -> _ -> 胎 b) 切 -> _ -> _ -> 含

a) 炎 -> 炉 -> 肩 -> 胎  

炎  EN, honou - flame

炉  RO - furnace, hearth

肩  KEN, kata - shoulder

胎  TAI - womb, uterus

b) 切 -> 召 -> 吟 -> 含  

切  SETSU, ki(ru), ki(reru) - cut

召  SHOU, me(su) - summon, wear, take

吟  GIN - sing, chant, recite

含  GAN, fuku(mu), fuku(meru) - hold, include, contain