认为“自然美是客观的，艺术美来自于自然美”这一观点的代表人物是（）
A.王夫之
B.叶燮
C.金圣叹
D.郑板桥
参考答案：A
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进行天然气置换，氮气置换空气时，测的氧气浓度低于（）时合格。
A、1％
B、2％
C、5％
