2143. What characters can be added here so that each character contains a part of the previous character? a) 弦 -> _ -> _ -> 終 b) 篤 -> _ -> _ -> 績

a) 弦 -> 畜 -> 累 -> 終  

弦  GEN, tsuru - string, bowstring

畜  CHIKU - animal raising, domestic animal

累  RUI - involvement, trouble, accumulation

終  SHUU, o(waru), o(eru) - end, finish

b) 篤 -> 駐 -> 素 -> 績  

篤  TOKU - serious, cordial

駐  CHUU - stop, reside

素  SO, SU - element, naked, simple

績  SEKI - achievements, spinning