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
[已经明白]
31. Statistics [indicate] that [approximate] every 22 years--within a range of 3 to 4 years--a major drought occurs in the United States
本来也是A,可以看看了词霸,尤其是approximate 也不怎么对阿?
(与单数动词连用)统计学:搜集、组织、解释数字数语的数字,尤指用样本分析总体的特征者
(used with a pl. verb)Numerical data.
(与复数动词连用)数字数据
40. [With more than half] the world's [annual yield] of 50 million tons of soy beans, [an important] source of protein, [is grown] in the United States.
是把with去掉吧?