Perhaps it's because our visual system evolved to be overly sensitive to even slight departures from the horizontal, to help keep us upright.
可能是由于视觉系统进化，使得我们对些微高于水平面的物体过度敏感，以此帮助我们挺直腰杆。
A rest shall be placed at the end of a row of books to keep them upright.
一排书的末端应放个支撑物以使书本保持直立。
To be safe, you should bring your seat backs to an upright position and keep your seat belts fastened during take-off and landing.
为了安全起见，在飞机起飞和着陆时，您应将座椅靠背调直，系好安全带。
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