如果之前没有过敏，现在出现过敏，原因在哪？

水准管气泡居中，表示（）。

A．竖轴铅垂

B．视准轴水平

C．竖轴水平

D．水准管轴铅垂

The following questions present a sentence, part of which or all of which is underlined. Beneath the sentence, you will find five ways of phrasing the underlined part. The first of these repeats the original; the other four are different. If you think the original is best, choose the first answer; otherwise choose one of the others.

These questions test correctness and effectiveness of expression. In choosing your answer, follow the requirements of standard written English; that is, pay attention to grammar, choice of words, and sentence construction. Choose the answer that produces the most effective sentence; this answer should be clear and exact, without awkwardness, ambiguity, redundancy, or grammatical error.

Unless the conservator can find a way to protect the photographs in the archives from light and air, these precious historical documents have deteriorated and will continue to do so both in color and in fine detail.（）

A. have deteriorated and will continue to do so

B. win continue to deteriorate

C. have been and will continue to deteriorate

D. will continue to deteriorate, as they already have,

E. will be deteriorating yet more

停炉大修时必须清扫煤粉仓，只有在停炉时间不超过三天，才允许煤粉仓内存有剩余煤粉。

按装载方式的不同，露天矿掘沟分为：平装车掘沟和（）。

A.铁路掘沟

B.公路运输掘沟

C.上装车掘沟

D.无运输掘沟

简述什么是货币替代。