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【单选题】
城市生态系统的脆弱性表现在：城市生态系统需靠外力才能维持，城市生态系统一定程度上破坏了自然调节机能，且( )。A.
食物链简化，自我调节能力小，营养关系倒置
B.
物质能量人口高度集中，营养关系倒置
C.
食物链简化，自我调节能力小，物质、能量、人口高度集中
D.
营养关系倒置
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举一反三
【单选题】_____ he thought he was helping us with the work, he was actually in the way．
A.
Since
B.
When
C.
Because
D.
If
【简答题】Although he thought he was helping us prepare the dinner, he was actually ________ the way. [A] in [B] by [C] off [D] on
【单选题】he thought he was helping us with the work, he was actually in the way.
A.
Although
B.
Unless
C.
Because
D.
When
【单选题】______ he thought he was helping us with the work he was actually in the way.
A.
Although
B.
Because
C.
If
D.
When
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