Why it cant be E. Why are you assuming that -1 is the first member in the set ? My unanimous choice was E. Am is wrong here ?
Because the question says, "which of the following MUST be in the set"
We only know for a fact that if a number is in the set, so is 2 plus that number. But that doesn't imply that the number is in the set BECAUSE of a previous number.
We know -1 is there. It implies that 1 is there, which pushes us to 3, which pushes us to 5. Those are "MUSTS". But it doesn't mean that -3 MUST be there.