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