Hi,teachers

Is there any difference between the two words ? Which one is more formal than the other one and which one do English natives use most?Why?

I admit [ but I can`t explain why] that "till" sounds to me as an unfinished word.Can I use them as follows?
a] I`ll be there till Friday .
b] I`ll be there until Friday.
or better
c] I`ll be there by Friday.

Are these sentences correct?

What is the difference [if any] among [a] , [b] and [c] ?


Thank you very much.