Hey folks!
I have a simple question on which I, as a non-native speaker, am having some difficulties.
We all know that according to parallelism, all parallel clauses should begin with the same preposition or relative clause, but I have seen so many confusing structures, I had to ask this question:

Consider the following sentence:
I lost my final paper, to which I have spent great effort and a great sum of money. (Disregard the meaning since I couldn't come up with a scenario )

As the meaning is clear here we opted to not repeat "to which". But when should one repeat the clause or the preposition?
I was also going the ask the 138th SC from OG13, but I figured it out, so I'll be glad if you could reaffirm me:

Most efforts to combat such mosquito-borne diseases like malaria and dengue have focused either on the vaccination of humans or on exterminating mosquitoes with pesticides.

A and dengue have focused either on the vaccination of humans or on exterminating
B like malaria and dengue have focused either on vaccinating of humans or on the extermination of
C as malaria and dengue have focused on either vaccinating humans or on exterminating
D as malaria and dengue have focused on either vaccinating of humans or on extermination of
E as malaria and dengue have focused on either vaccinating humans or exterminating

Since the paralellism should be between two clauses of either.... and or.... the OA is E. But what if C was something like "as malaria and dengue have focused on either vaccinating humans or exterminating" (without the 2nd "on")
Would that be also true?

I know I could have been clearer with the question I asked, but I think the experts here will understand what I mean.
I gladly await your response in my final day before the GMAT

Thanks in advance,
Cinar