问题
选择题
| It's necessary for human beings to breathe _______ and plants can produce it. |
A. CO2 B. H2 C. O2 D. N2 |
答案
答案：C
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| It's necessary for human beings to breathe _______ and plants can produce it. |
A. CO2 B. H2 C. O2 D. N2 |
答案：C