The success of the program to eradicate smallpox has stimulated expert

Oh man, this question has so many hidden layers to understand! I got this wrong while solving but here is what I have understood after perusing many expert replies:

A
- past perfect use is ok here.
- However, the use of "what" means something very specific, and hence the usage is not great. eg: I want to study what I love. I want to study something I love. both are correct, but the 1st sentence means something very specific and second is generic.
- "the other infections such as x, y" is wrong. Copying Ron's response (from Manhattan Prep forum) below:
in general, you shouldn't have both of these -- i.e., you shouldn't have "the" + noun + "such as", regardless of what else is around it.
"the" implies a definite, specified item or set of items, while "such as" implies items that are unspecified but similar to whatever "x" is. so, basically, the ideas of "the" and "such as" are contradictory.
ex:
you should do the exercises in the book.
--> it wouldn't make sense to have "such as" here, because the book will actually contain a specific group of exercises.

you should do exercises such as those in the book.
--> this means that you can basically do [i]any exercises of the same kind, regardless of whether they actually appear in the book. since this isn't a definite set of exercises, it wouldn't make sense to use "the" here.


B and E such... like wrong

C correct

D
- comma before 'of' doesn't make sense
- Difference between "not considered a previous possibility" and "not previously considered possible"
1st means there was a possibility in the past but now better control and eradication of infections is not considered a previous possibility. so is it a current possibility or future possibility?
2nd means better control and eradication of infections was not considered possible.

Hope this helps!