或者，你可以选两段困难度相当的段落，在结束前，一段快弹，另一段慢弹，然后隔天比较它们的差异。
Or you can practice a passage fast only and another passage (of the same difficulty) slowly at the end and compare them the next day.
油井压裂选井选层是一项复杂有难度的技术。
The selection of wells or layers for fracturing is an intricate task.
在女儿的这堂钢琴课上，老师拿出一本吓人的包括所有的按难度排列的钢琴课曲目琴书来选一首上课用的新曲。
During this lesson, the teacher took out a frayed book with all the lesson pieces arranged according to difficulty, for choosing a new piece to study.
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