Why was Lily given a chance to survive?

[grangerohini]

No: HPFGUIDX 161455

I've been reading Harry Potter book 1 ..and I've already read all books 3
times now . I've got a question that in book 1 in the end when
Dumbledore said to Harry that he can ask anything but Dumbledore will
answer only those questions which he will think suitable for Harry
to hear (he said something like that) and Harry's first question was
about Lily that why didn't Voldy kill Lily in the first place. Why did
he ask her to step aside? But Dumbledore didn't answer, right? He said he
will answer him later but I think he didn't in all the books ..so
what is the answer? Dumbledore told Harry everything but just that
question of Harry's.
What's the answer? How and when will Dumbledore answer it?

grangerohini