*Soretomo* means "or" and connects two alternatives.
_"Would you like water __or__ tea?"_
*Mizu ga ii desu ka, __soretomo__ ocha ga ii desu ka?*
(Also can be expressed saying: *Mizu ni shimasu ka, soretomo ocha ni shimasu ka?*)
Although *'ka'* also means_ "or" _YOU CANNOT SAY:
x *Mizu ga ii desu ka, __ka__ ocha ga ii desu ka?*
BUT YOU COULD SAY:
*Mizu ga ii desu ka, __soreka__ ocha ga ii desu ka?*
:idea: *'ka'* often follows question words immediately, if it's not the end of a sentence. Such as in:
*nani > nanika* (what > something)
*dare > dareka* (who > someone)
*itsu > itsuka* (when > sometime/someday)
*naze > nazeka* (why > for some reason)
*dore > doreka*(which > one or another)
In such instances, the compound word is not a question but rather refers to an indefinite person, place, thing, number, time.
:idea: ALSO, note that the particle *ka* between two nouns (and not at end of sentence) means "or". *Ka* when repeated after two nouns in sequence means "either ~ or ~" expressing a choice between two alternatives.
*Migi ka hidari ka, dotchi? * (Right or left, which?)