膨胀机按结构分类有()形式。 A.活塞式膨胀机 B.透平式膨胀机 C.增压透平式膨胀

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问题:

膨胀机按结构分类有()形式。

A.活塞式膨胀机

B.透平式膨胀机

C.增压透平式膨胀机

D.旋转式膨胀机

考点:制氧工考试制氧工综合练习制氧工综合练习题库
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Last year college students in America spent an estimated $700 on textbooks on average. The National Association of College Stores reported more than five billion dollars in sales of textbooks and course materials.

The association spokesman Charles Schmidt says electronic textbooks now just make up 2%~3% of sales. But he says that is expected to reach 10%~15% by 2012.

Online versions(版本) are now available for many of the most popular college textbooks. An e­textbook can cost half the price of a new print textbook. But students usually lose access to them after the end of the term. And the books cannot be placed on more than one device(设备),so they are not easy to share.

So what do students think of e­textbooks?Administrators at Northwest Missouri State University wanted to find it out. Earlier this year they tested them with 500 students in 20 classes.

The university is unusual. It provides laptop computers for all 7,000 of its full­time students. It does not require students to buy their textbooks either. They rent them to save money. The school aims to save even more by moving to e­textbooks.

The students in the survey reported that downloading the books from the Internet was easy. They liked the idea of carrying lighter backpacks. And 56% said they were better able to find information.

But most found that using e­textbooks did not change their study habits. And 60% felt they read more when they were reading on paper. In all,almost half the students said they still liked physical textbooks better.

But the survey found that cost could be a big influence.55% said they would choose e­textbooks if using them meant their textbook rental fee would not increase.

Roger Von Holzen heads the Center for Information Technology in Education at Northwest Missouri State University. He tells us that administrators are disappointed with the e­textbooks now available because the majority are not interactive(交互式的).

He thinks growth will come when more digital books include video,activities,games and other ways to interact with the information. The technology is improving. But for now,most of the books are just words on a screen.

小题1:E­textbooks are not better than paper books in that________.

A.they cost more money

B.they’re difficult to carry

C.they’re not convenient to share

D.they can’t be downloaded from the Internet小题2:How many surveyed students think paper books make better reading?

A.60%.

B.56%.

C.55%.

D.50%.小题3:It can be inferred from the text that in the future________.

A.digital books will be more popular

B.the digital books available need improvement

C.free digital books are available online

D.digital books will replace print textbooks

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熔渣的物理性能是指熔渣的()、()、()、脱渣性和透气性。

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电弧喷涂喷嘴

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可以在二十五度以上陡坡地开垦种植农作物。

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广东鹤兰德州头机械化养猪场的农业地域类型属于()

A.商品农业和密集农业

B.畜牧业和粗放农业

C.自给农业

D.混合农业

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