求助0410的几道语法题，谢谢！

uspresidentUID: 2123602

错了10题丧失信心的孩子提问，希望大家解答，谢谢！
15. A challenging new area in inorganic chemistry is ---- the role of transition metals in the biochemical catalysts called enzymes.
     (A) that of understanding
     (B) to have understanding
     (C) the understanding
     (D) understanding that
选A怎么理解？我选D,3ks.
20. Initially introduced in 1852, the gyroscope consists a spinning device, usually in the form of a wheel, that exhibits strong angular momentum.
B怎么改呢？为什么错呢？谢谢。
34. After the United States became independent, the cure of more fertile lands drew steadily New Ennglanders into the Ohio Vallye and the British colony of Upper Canada.
C是不是就换个位子就可以了？
39. By 1810 the 23 towns of Hampshire County, Massachusetts, had reached a remarkable uniform of economic development as well as population density.
最后一个为什么错呢？怎么改呢？谢谢！

hargenUID: 194793

第2题是consists of
其他三道同错
同问
我错了6个，也好郁闷
昨晚还做过一份全对的，反差咋这么大呢？？？