单项选择题

It can be inferred from the passage that （）.

A．a stepladder used to have four legs.
B．it is dangerous to use tools
C．a tripod is safer in a tool design
D．workers are safer on production lines

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单项选择题
Of all the oceans in the world, the Indian Ocean contains more than（）

A．5 per cent of the world’s water
B．15 percent of the world’s water
C．20 percent of the world’s water
D．50 percent of the world’s water
单项选择题
When some of the stamps are difficult to find, （）

A．nobody will try to get them
B．they can become very valuable
C．everybody would like to buy them
D．they will be worthless
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