ZHUJ908 Wrote:but you did not mention why "both"should be omited meaningly.
...because there is no such reason.
for the purposes of this exam, "both X and Y" and "X and Y" are equivalent.
the difference is purely stylistic. "both" is included (1) if "X" and "Y" are long (= long enough to make the sentence difficult to read without "both"), or (2) for rhetorical reasons, such as emphasis.
style issues are not tested on this exam, so you only have to know that...
..."X and Y" is a legitimate parallel structure,
..."both X and Y" is also a legitimate parallel structure.
end of story.