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【单选题】

Mr. Leslie, who _____ in Taiyuan for three years in the 1980's, is now a well-know lawyer in London.

[ ]

A.
has worked
B.
worked
C.
was working
D.
would work
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参考解析:
.
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【单选题】污染物会造成生态系统的破坏,其主要原因是( )

A.
污染物的成分不同于自然物质
B.
污染物在生态系统中不易被细菌分解
C.
只有人类才可以利用这些污染物
D.
污染物仅对生态系统产生影响,不影响人类