2302. What character can be added to both of these characters to give other characters? 北_ -> _旦

Add 月:北+月 月+旦   

北  HOKU, kita - north

旦  TAN, DAN, akira, tadashi, asa - morning 

月  GETSU, GATSU, tsuki - moon, month

背  HAI, se, somu(ku), somu(keru) - back, height, act contrary to, avert

胆  TAN - gallbladder, courage