the technique behind getting this problem right involves knowing the concept of Ellipsis.
The omission of one or more items from a construction in order to avoid repeating the identical or equivalent items that are in a preceding or following construction is known as Ellipsis
For eg: I've been to Paris, but they haven't.
This example when expanded actually means, I have been to paris but they have not been to paris.
Here 'Been to paris' is implied
Now lets deal with our question. the causes of crime explains why most people do not commit crimes conversely why so many of those ('those' refers to people here) not so exposed --- what should we fill in.
why most people do not and why most people not exposed do.
Notice the words in red. only when you have donot- do, the sentence maintains parallelism.
have, has, shall and could do not maintain this parallelism. Hence they can be eliminated at the first shot itself, even without giving them much thought.
Hope this helps!
