They are not related to one another as a series of textbooks, graded in difficulty or in the technicality of the problems with which they deal .
它们各自之间没有联系，不像一套教科书那样，按照书中问题的难易程度，或专业的深浅分级别类。
The texts should be graded in order of difficulty .
课文要按难易程度编排。
These will not be graded, but the questions appearing on the problem sets will be very similar in depth and difficulty to those appearing on the exams.
习题将不会计分，但是习题在深度和难度上会和测验中的题目相似。
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