91.By measuring the proportion of heavier hydrogen and oxygen isotopes along a strand of air,scientists can construct a geograghic timeline.(09.6 Passage2)

　　分析主干：scientists can construct a geograghic timeline

　　动名词measuring作by的宾语，measuring还自带宾语the proportion。介词短语of heavier...isotopes中，hydrogen和oxygen是并列成分，共同修饰isotopes,along...介词短语也是isotopes的后置定语。

　　译文：通过测量一缕头发不同生长时期重量更大的氢原子和氧原子同位素的比例，科学家就可以绘制出一幅地理活动时间图表。