GMAT Sentence Correction (SC) | Re: Like the Dreyfus affair at the century and the Sacco-Vanzetti trial in

This is whathttp://www.merriam-webster.com/dictionary/(all)%20the%20more%20so says about "the moreso":

Used to say why something (such as an attribute or quality) applies to a greater degree or extent <The play was impressive—(all) the more so because the students had written it themselves .

As a non-native speaker, I was also totally clueless on this one.
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