Efficient' is a slightly ambiguous term. In terms of storage requirements, the most efficient solution would be a bit-field - but that will result in very inefficient queries - and will be difficult to adapt to changes in the data model. If it were me I'd just normalise it.
You can use as a serialize or JSON like {a1:0, a2:1,a3:0} . When select this data ..decode it again... I prefer this way...
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.