Trees can help ______ the air to keep it ______.

A: clean, clean
B: cleaning, cleaning
C: to clean; cleaning
D: cleaning; clean
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科目：初中英語 來源：期末題 題型：單選題

Trees can help ______ the air to keep it ______.
[     ]
A: clean, clean
B: cleaning, cleaning
C: to clean; cleaning
D: cleaning; clean

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