下列关于丙烯(CH3-CH=CH2)的说法正确的是 [ ] A.丙烯分子有8个σ

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问题:

下列关于丙烯(CH3-CH=CH2)的说法正确的是 [ ]

A.丙烯分子有8个σ键,1 个π键

B.丙烯分子中3个碳原子都是sp3杂化

C.丙烯分子不存在非极性键

D.丙烯分子中3个碳原子可能在同一直线上

考点:σ键、π键杂化轨道理论(中心原子杂化方式)极性键、非极性键有机物分子的空间构型
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Where ___ you be this time tomorrow?

A.will

B.being

C.shall

D.do

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膝关节脱位行血管造影的指征是()

A.双下肢动脉搏动不对称

B.常规检查

C.患肢未触及动脉搏动

D.超声检查可替代,无需行血管造影

E.MRE可替代,无需行血管造影

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当投影面与轴测投影面倾斜时,所得到的具有立体感的投影,称为正轴测图。()

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Electronic mail has become an extremely important and popular means of communication.
The convenience and efficiency of electronic mail are threatened by the extremely rapid growth in the volume of unsolicited commercial electronic mail. Unsolicited commercial electronic mail is currently estimated to account for over half of all electronic mail traffic, up from an estimated 7 percent in 2001, and the volume continues to rise. Most of these messages are fraudulent or deceptive in one or more respects.
The receipt of unsolicited commercial electronic mail may result in costs to recipients who cannot refuse to accept such mail and who incur costs for the storage of such mail, or for the time spent accessing, reviewing, and discarding such mail, or for both. The receipt of a large number of unwanted messages also decreases the convenience of electronic mail and creates a risk that wanted electronic mail messages, both commercial and noncommercial, will be lost, overlooked, or discarded amidst the larger volume of unwanted messages, thus reducing the reliability and usefulness of electronic mail to the recipient. Some commercial electronic mail contains material that many recipients may consider vulgar or pornographic in nature.
The growth in unsolicited commercial electronic mail imposes significant monetary costs on providers of Internet access services, businesses, and educational and nonprofit institutions that carry and receive such mail, as there is a finite volume of mail that such providers, businesses, and institutions can handle without further investment in infrastructure. Many senders of unsolicited commercial electronic mail purposefully disguise the source of such mail.
Many senders of unsolicited commercial electronic mail purposefully include misleading information in the messages’ subject lines in order to induce the recipients to view the messages. While some senders of commercial electronic mail messages provide simple and reliable ways for recipients to reject (or "opt-out" of) receipt of commercial electronic mail from such senders in the future, other senders provide no such "opt-out" mechanism, or refuse to honor the requests of recipients not to receive electronic mail from such senders in the future, or both.
Many senders of bulk unsolicited commercial electronic mail use computer programs to gather large numbers of electronic mail addresses on an automated basis from Internet websites or online services where users must post their addresses in order to make full use of the website or service.
The problems associated with the rapid growth and abuse of unsolicited commercial electronic mail cannot be solved by the government alone. The development and adoption of technological approaches and the pursuit of cooperative efforts with other countries will be necessary as well.

Which of the following is NOT true about unwanted e-mail

A. It costs money to receive them.
B. It’s free to store them.
C. It takes time to access them.
D. It takes time to throw them away.

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电缆采用铅包的缺点是()

A.电阻率高

B.不易焊接

C.不易加工

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