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。
2
The texts should be graded in order of difficulty .
课文要按难易程度编排。
3
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.