as 作介词表示作为，function as sth，作牙齿用，ok
as作连词，表示像……一样，因为，当……时，function as teeth do，像牙齿一样作用，这里应该就是当作牙齿用的意思
do sth for sb,为某人做某事，do sth for doing sth，表用途或原因，use the knife for cutting; thank u for coming.
C，An argument concerning the present-day uses to which historical maps of Native American lands are put=An argument concerning the uses that historical maps of Native American lands are put to uses，讨论印第安地图在现在的用处；native american本土美国人，印第安人
还是要理解句字意思：The sentence, as stands, is confusing. It implies that the 'small cars' are now more fuel efficient than they were in the past. That is the manufactures took an old small car and said, "hey let's make this more efficient." Clearly, however, the small cars today are different from those in the past.we can omit (A), (B).
C gives us a strong reason why reclamation has gotten cheaper. The nature of the mines has changed, making it less difficult to reclaim them.
B looks good EXCEPT look very carefully at the wording of the argument. It says "average cost of a coal mine". Creating fewer coal mines does not matter to this argument, because it is average cost PER mine reclaimed that we are concerned about.
In the time since the reclamation regulations were put in place, the average reclamation cost of a coal mine has gone down, even though there has been no improvement in technology. How can this be?
Possible reason: Even though technology has not improved, some other factor has changed that permits less costly reclamation.
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因为downtown缺少business需要的现代化办公楼，很多商户离开了Goal: 为了吸引business回到downtown Plan: 在downtown建立现代化办公楼，以及提供减免后的税收
choice c, 郊区税率比市区税率低。correct-> 说明downtown即使配置了办公楼依然无法和郊区竞争