A parallel sentence mentions the same grammatical expression twice. I would go with "D". 'Passed' and 'Demonstrated' are both past tense words that express the same past tense grammatical idea.
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Answer:
While comparing voluntary and mandatory recycling programs, the benefit of
educating communities must not be overlooked.
Although the environmental benefits of mandatory recycling are real, the costs for
implementation of such recycling programs are significant.
Explanation:
Probably not there are a lot of people that do weird things
