The success of the program to eradicate smallpox has stimulated expert

Hey Charles GMATNinja, having hard time understanding the past perfect on this sentence. I discarded option C and all options with "had"/past perfect as I could not find a time market, I was having hard time to understand the sequence using the past perfect. Also, saw many problems with Present Perfect and Past Perfect combo to be identified as "Wrong". Can you provide some clarity on the usage of Past Perfect in this question? Thanks!

Although I got this one correct, i am not sure about the use of past perfect tense in option C. We have "previous" so i believe, using past perfect is not ideal here.
Please share your valuable insight here.
Thanks

In all these options. Doesn't the marker 'previously' make the term 'had' redundant?. I got this in my mock when I wasn't left with much time so I went with 'D'. 'B' & 'E' were anyways out cause of the word 'like'.

Prithvi08

In that vein, you could argue that any use of "previously" is redundant, because you'll always be writing about something in the past, and will therefore be using the past or past perfect tense. We could say the same for modifiers such as "also" or "actually," which don't really add much in the way of meaning. However, these modifiers can increase clarity by emphasizing the intended meaning. We use "previously" to emphasize a change: previously things were this way, and now/later on they are different.

The success of the program to eradicate smallpox has stimulated experts to pursue what they had not previously considered possible−better control, if not eradication, of the other infections such as measles and yaws.

Option elimination -

(A) what they had not previously considered possible−better control, if not eradication, of (correct usage - usage of what to introduce a relative clause to modify the noun experts) the other infections such as - the problem is "the" The means only limited to "the other infections." What are those specific other infections - measles and yaws?. The sentence actually means other infections such as measles, yaws bla bla ....

(B) what they had not previously considered a possibility−better control, if not eradication, of such infections like - easy one. Like is wrong.

(C) something they had not previously considered possible−better control, if not eradication, of such infections as - the is gone, even other is gone. Better.

(D) something not considered a previous possibility−better control and perhaps eradication, of other infections such as - "a" is a problem. "and" is a problem. The actual meaning is better control (if not eradication). Here it says: better control and eradication - either we control or eradicate ....how do we do control and eradication simultaneously? Wrong. Comma before "of" not required.

(E) the possibility of what they had not previously considered possible−better control and possibly eradication of infections like - same "and". Like is wrong.