然而，他们认为那些睡眠时间不超过8小时的学生在回忆旧知识、学习新课程和完成高质量工作方面面临着最大困难。
Yet they rated the students who have received eight hours or less as having the most trouble recalling old material, learning new lessons, and completing high-quality work.
他们还选择了超过1200个不活跃的星系，它们的距离和亮级都相同，这样可以保证图像的质量相同。
They then chose more than 1,200 inactive galaxies at the same distance and brightness levels to ensure all images were of the same quality.
为了保证教学质量，坚持编制不超过十五人的小班授课。
There are no more than 15 students in every class so as to guarantee the teaching quality.
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