2. True friendship is pure, not a trace of dirt doping. 真正的友谊是纯洁的，不会掺杂一丝污垢。
3. The composition of oxygen isotope is a good tool to trace the origin and formation environment of sulfate. 氧同位素组成是一个非常有效的示踪硫酸盐的起源和形成环境的工具。
英英解释
n.
1. a just detectable amount
2. an indication that something has been present
3. a suggestion of some quality
4. a drawing created by superimposing a semitransparent sheet of paper on the original image and copyin
5. either of two lines that connect a horse's harness to a wagon or other vehicle or to a whiffletree
6. a visible mark (as a footprint) left by the passage of person or animal or vehicle v.
1. follow, discover, or ascertain the course of development of something
2. make a mark or lines on a surface
3. to go back over again
4. pursue or chase relentlessly
5. discover traces of
6. make one's course or travel along a path; travel or pass over, around, or along
7. copy by following the lines of the original drawing on a transparent sheet placed upon it; make a tr
8. read with difficulty
