Maxwell’s theory of Electromagnetism in 1865 was the first “unified field theory”, with Electroweak in 1961 only being the other, because no further theory has united the electroweak field with either the Strong (Hadronic) force or Gravity. A. with Electroweak in 1961 only being the other, because B. with Electroweak in 1961 being the only other, because C. and Electroweak in 1961 only was the other, because D. and Electroweak in 1961 only being the other, as E. and Electroweak in 1961 the only other, as For a discussion of dropping common words in parallel structure, as well as a full explanation for this question, see: https://magoosh.com/gmat/2013/dropping-c ... -the-gmat/ Mike

GMATNinja , KarishmaB , DmitryFarber In this question we have "having united", isnt having united a present participle and having+united basically shows an action that is done, correct? so how is it correct here?

I don't know the question to which you are referring because this one doesn't have 'having united...' But note that 'having united' would be perfect participle formed by combining ‘having’ with a past participle. It shows that an event happened before another. Examples: Having seen the play before, I decided to skip it today.

arry232 Are you referring to the fact that we're using "has united" to describe an action that HASN'T happened? That's absolutely fine to do. (In fact, it's what we're saying when we saying the action HAS NOT happened.) If you are referring to something else, let us know.

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