2644. What characters can be added here so that each character contains a part of the previous character? a) 針 -> _ -> _ -> 念 b) 章 -> _ -> _ -> 球

a) 針 -> 古 -> 吟 -> 念  

針  SHIN, hari - needle

古  KO, furu(i) - old

吟  GIN - sing, chant, recite

念  NEN - thought, concern

b) 章 -> 古 -> 呈 -> 球  

章  SHOU - chapter, badge, mark

古  KO, furu(i) - old

呈  TEI - offer, present, exhibit

球  KYUU, tama - ball, sphere