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。
The texts should be graded in order of difficulty.
那要塞易于成为空袭的目标，很难防守。
The texts should be graded in order of difficulty.
那要塞易于成为空袭的目标，很难防守。
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