Where did the fish choose to stay in the unique dish tank?
A. They preferred to stay in the clean section.
B. They preferred to stay in the empty section.
C. They preferred to stay in the natural section.
D. They preferred to stay in the unnatural section.
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What did the researchers do after they gave the fish shots?
A. They gave all the fish in the fish tank some painkilling shots.
B. They put painkilling medicine in the water of both sides of the fish tank.
C. They put painkilling medicine in the water of the natural side of the fish tank.
D. They put painkilling medicine in the water of the empty side of the fish tank.
What happened after the researchers put painkilling medicine in the water?
A. Fish that received the painful shot stayed in the natural section.
B. Fish that received the painful shot moved into the empty section.
C. All of the fish stayed in the natural section with plants and rocks.
D. All of the fish moved into the empty section with painkilling medicine.
What does this clever study suggest?
A. Fish can’t feel pain when they receive shots.
B. Fish can experience pain and try to reduce it.
C. Fish like to swim in an unnatural environment.
D. Fish like to swim in a less pleasing environment.
刚体定轴转动，为了消除轴承动反力，其转轴必须是（ ）。
A. 过刚体质心的转轴；
B. 转轴垂直于刚体质量对称面；
C. 惯性主轴；
D. 中心惯性主轴。
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