单项选择题

It can be concluded from the passage that the immune systems of animals ________.

A．can be weakened by conditioning
B．can be suppressed by drug injections
C．can be affected by frequent doses of saccharin
D．can be altered by electric shocks

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单项选择题

Laudenslager's experiment showed that the immune system of those rats who could turn off the electricity ________.

A．was strengthened
B．was not affected
C．was altered
D．was weakened

单项选择题

The passage tells us that the most probable reason for the death of the mice in Ader's experiment was that ________.

A．they had been weakened psychologically by the saccharin
B．the sweetener was poisonous to them
C．their immune systems had been altered by the mind
D．they had taken too much sweetener during earlier conditioning