2032. What character can be added to both of these characters to give other characters? 其_ -> _是

Add 土:其+土 土+是   

其  shi, so - that

是  ZE - right, correct, just

土  DO, tsuchi - earth, soil

基  KI, moto, motoi - basis, foundation

堤  TEI, tsutsumi - bank, embankment, dike