单项选择题

It is important to study polar bears to assess the health of the ecosystem on the Arctic, because（）

A. they are on the edge of extinction.
B. they are on the top of the ecosystem.
C. the food chain on the Arctic depends on them.
D. there are no other big animals on the Arctic.

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单项选择题
According to the passage, the trip to Los Angeles is not complete unless（）

A. people have seen the word "Hollywood" on a hillside.
B. people have eaten the famous fruits there.
C. people have s been to the beautiful mountains and beaches.
D. people have visited the motion picture and television industries.
单项选择题
The expected life-span of Beijing residents has gone up by （） compared with that a decade earlier.

A. 1.5 years
B. 1.4 years
C. 1.2 years
D. 1.1 years
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