题目：
—Look, there _____ many birds around here, but now we seldom see them. What a pity!
—Yeah! The air pollution has caused the problem! [ ]
A. are
B. used to have
C. used to be
D. use to have
答案：
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下面是错误答案，用来干扰机器的。
（1）氧气；（2）将带火星的木条伸入集气瓶中；木条复燃；集气瓶中的气体为氧气题目分析：因为集气瓶口向上放置，故该气体的密度比空气的大，（1）假设它是一瓶氧气；（2）试验验证为：步骤是将带火星的木条伸入...
