苷类酸催化水解的机制是（） A.苷键原子先质子化，然后与苷元之间键断裂生成阳碳离子，

《中 * * 、国务院关于加快水利改革发展的决定》提出，现代农业建设不可或缺的首要条件是（）。

A.交通

B.设备

C.水利

D.能源

三个阻值相等的电阻串联后的等效电阻是180Ω，那么它们并联后的等效电阻是______Ω．

The questions in this group are based on the content of a passage. After reading the passage, choose the best answer to each question, Answer all questions following the passage on the basis of what is stated or implied in the passage.

Forget hostile aliens. According to a forthcoming book by noted astrophysicist Egbert Larson, the intrepid humans who first attempt interstellar space travel will face far more daunting challenges before they ever meet the Little Green Men.

Larson begins with the problem of relativistic time dilation. If you travel all the way to Alpha Centauri, you’d like to come back and tell your friends about it, right It’s not too likely to happen, though. If Mr. Einstein was right about relativity--and we’re not going to say he wasn’t--then time slows down when you approach the speed of light. A person traveling at any velocity near the speed of light will age only days for every week, month, or even year that passes on earth. Relativity does not present a problem for interstellar space travel, per se, but it does mean that interstellar civilizations or even just interstellar communications will require a mind-boggling amount of calendar juggling.

Did we mention that you’d have to travel at near the speed of light That’s because the distance between stars is so vast that even if you could travel at the speed of light--which, Larson reminds us, you can’t--it would take more than four years to reach our closest star neighbors, Alpha Proxima and Alpha Centauri, and decades or centuries to reach the other stars in our "immediate neighborhood." And if you tried to accelerate directly to the speed of light like they do in the movies, you’d be instantly splattered on the back of your theoretical spacecraft. Achieving anything close to light speed will require sustained accelera- tion at a level that human bodies can withstand--say, a crushing two gravities--for over a year. Better hope somebody brings some chips.

Speaking of chips, food is going to be a problem. Since it is economically, if not physically, impossible to accelerate 200 years’ worth of food to nearly the speed of light, and since you’re not likely to find any grocery stores along the way, someone will have to figure out how to make food in space. Keeping a crew alive on the way turns out to be the trickiest part of all. Once you’ve got the nearly impossible physics of space travel worked out, you still have to figure out the chemistry and biology of keeping your air and water clean and keeping your crew fed and safe from radiation and infection, and--did we mention the 200 years--you’ll probably need several generations of crew members to complete the trip. Ever been on a bus for more than 24 hours It’s not a pretty picture.

We applaud Larson for his insightful writing and his scrupulous attention to scientific detail. For those of you seeking a cold, hard look at the reality of interstellar space travel, this is a stellar read. But be warned: Larson doesn’t let you down gently. For those of you sincerely hoping to beam up with Scotty--and you know who you are--you might want to give this one a pass.

According to the passage, which of the following will present the most difficult challenge for humans attempting interstellar space travel ？（）

A. Achieving velocities near the speed of light

B. Withstanding the acceleration necessary for traversing interstellar distances

C. Maintaining clean air and water on a journey that could last centuries

D. Accommodating the effects of relativistic time dilation

E. Enabling the humans on board to survive during the journey

建设工程风险评价的主要作用在于确定( )。

A．风险损失值的大小

B．风险发生的概率

C．风险的相对严重性

D．风险的绝对严重性

关于MPLS转发流程中Ingress节点转发的描述正确的是（）。

A.Ingress节点收到数据包之后会首先查看ILM表，查找Tunnel ID

B.根据ILM表的Tunnel ID找到对应的NHLFE表项，将LFIB表项和NHLFE表项关联起来

C.查看NHLFE表项，可以得到出接口、下一跳、出标签和标签操作类型，标签操作类型为Push

D.在IP分组报文中压入获得的标签，并根据QoS策略处理EXP，同时处理TTL，然后将封装好的MPLS分组报文发送给下一跳