单项选择题

What is the passage mainly about

A．How biochemical changes occur in the human brain.
B．Why people should keep active not only physically but also mentally.
C．How intellectual activities influence brain-cell health.
D．Why people should receive special mental training as they age.

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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