单项选择题

Older people are generally advised to______.

A．keep fit by going in for physical activities
B．keep mentally active by challenging their brains
C．maintain mental alertness through specific training
D．maintain a balance between individual and group activities

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单项选择题

Which of the following describes the main idea of the passage

A．Computers are stupid and inefficient.
B．Computer errors are basically human errors.
C．Computer can help department stores update the price list.
D．Supermarket price errors are often made through dishonesty.

单项选择题

According to the passage, if the price in the machine is too high, the cause(s) may be______.

A．a computer error
B．carelessness
C．dishonesty
D．B or C