单项选择题

The causes of sleep problems in older people remain unidentified.（）

A、Right
B、Wrong
C、Not mentioned

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单项选择题
Which of the following is NOT true of mental retardation（）

A．It is not curable
B．Its victims are usually born with it
C．Its causes are not fully identified yet
D．It can spread among family members
单项选择题
Many older physicians in the U. S. view the work ethic of their younger counterparts（）

A．with appreciation
B．with disapproval
C．with jealousy
D．with indifference
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