One explanation given by scientists for the crop circles is that they are made by ______.
A. airplane crashes
B. air movement
C. unknown flying objects
D. new farming techniques
Which of the following may prove that the crop circles are not made by man
A. The farmers couldn't step out of the field
B. The farmers couldn't make the circles round
C. The farmers couldn't leave without footprints
D. The farmers couldn't keep the wheat straight up
What does the author suggest working women with families do
Attend cooking lessons with your family
B. Prepare simple meals for your family before a trip
C. Encourage your family to have fun while you are away
D. Help your family to learn to prepare food for themselves
负强化是指（）。
A. 呈现一个刺激以增加一个反应发生概率的程序
B. 呈现一个刺激以降低一个反应发生概率的程序
C. 撤消一个刺激以降低一个反应发生概率的程序
D. 撤消一个刺激以增加一个反应发生概率的程序
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