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
From the passage we know that joints and muscles perform best when______.
A．they are twisted and stretched
B．they are in their normal position
C．they are tested with a human" guinea pig"
D．they are tested with electronic instruments
单项选择题
To parents who take the responsibility for children's expenses, the cost of living increases because______.
A．children attend school for more years
B．children are better dressed
C．children spend more money on entertainment
D．all the above
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