1901. What character can be added to both of these characters to give other characters? 戸_ -> _包

Answer:

戸  KO, to - door

包  HOU, tsutsu(mu) - wrap

Add 月:戸+月 月+包 

月  GETSU, GATSU, tsuki - moon, month

肩  KEN, kata - shoulder

胞  HOU - sac, sheath, placenta