be carefull my friends.
not only...but also is used to show the in-the-same- direction content between 2 clauses.
however, "but also" can be used ALONE to show contrast and means "but"
gmatprep presents a question in which "but also " is used ALONE to test whether we know the contrast.
in this question, the meaning is same-direction content between 2 clauses, so, the couple "not only...but also" is used
this is called meaning based analysis which assumes that the wrong answer choice is grammatical and needs to be proved wrong in meaning.
this means "but also" in b and c are grammatical but wrong in meaning.
if you think that "but also" in b and c are wrong because "not only" dose not go with them, you are wrong because "but also" can be used without "not only"
that is my thinking.
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