运用社会分层理论，谈谈我国改革以来社会分层结构的变化。
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The text tells us that the universe______.
A. was formed by a random observer
B. has been in existence for an infinite amount of time
C. has undergone change over time
D. is constantly expanding
某些儿童出现吃土、煤渣等异嗜癖是营养性缺铁的症状。()
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