I believe that in the last paragraph the authors statements:

"...-indeed, these differences will be revealed with more authority in the absence of conscious intention."

"She points out..."

Were what I would point to when deciding between C and D. It is very subtle, but it paints Gluck in a positive light.

One thing is that the conclusion is not a conditional that relies on "if all types of bacteria were rather similar." We know for a fact that bacteria are not similar and it is used as evidence to support the conclusion.

I feel like I knew the answer, but talked myself out of it because I thought "just because they have shown this doesn't mean this will be true in the future."

However, I need to remember the threshold for strengthen questions does not have to absolutely prove it like a sufficient assumption question might.