设全集U={2，3，a2+2a-3}，A={2，b}，∁UA={5}，则a=__

有机化学中有多种同分异构现象，其中有顺反异构，例如2-丁烯有下列两种顺反异构体： 

(A)中的两个-CH₃在双键的同一侧，称为顺式；

(B)中的两个-CH₃在双键的不同侧，称为反式。

若把其中一个-CH₃ 换成-C₂H₅，只要在双键同侧，便称顺式，否则为反式。

据此苯丙烯(C₉H₁₀)具有(C)和(D)两种位置异构体，其中(C)具有顺式(E)和反式(F)两种异构体，请写出它们的结构简式_____________。

附生效条件的合同生效时点的说法错误的是：( )。

A．合同有效成立时生效

B．条件成就时生效

C．不正当促成条件成立的合同不生效

D．不正当阻碍条件成就的合同生效

氨和联氨(N₂H₄)是氮的两种常见化合物，在科学技术和生产中有重要的应用。根据

题意完成下列计算：

联氨用亚硝酸氧化生成氮的另一种氢化物，该氢化物的相对分子质量为43.0，其中氮

原子的质量分数为0.977，计算确定该氢化物的分子式为      。该氢化物受撞击则完全分解为氮气和氢气。4.30 g该氢化物受撞击后产生的气体在标准状况下的体积     L。

联氨和四氧化二氮可用作火箭推进剂，联氨是燃料，四氧化二氮作氧化剂，反应产物

是氮气和水。由联氨和四氧化二氮组成的火箭推进剂恰好完全反应生成72.0 kg水，推进剂中联氨的质量为        kg。

氨的水溶液可用于吸收NO与NO₂混合气体，可消除氮氧化合物对环境的污染。请分

别写出有关的反应方程式为：                     ；                       。

风湿性心脏瓣膜病患者发热两周，疑有感染性心内膜炎，下列哪项体征对诊断最有帮助（）

A.进行性贫血

B.脾大

C.杵状指

D.主动脉瓣区突然出现舒张期杂音

E.皮肤黏膜出血点

A Trip to the Forest

One day Bob took two of his friends into the mountains. They put up their tents and then rode off to a forest to see how the trees were growing.
In the afternoon when they were about ten kilometres from their camp, it started to snow. More and more snow fell. Soon Bob could hardly see his hands before his face. He could not find the road. Bob knew there were two roads. One road went to the camp, and the other went to his house. But all was white snow. Everything was the same. How could he take his friends back to the camp
Bob had an idea. The horses ! Let the horses take them back ! But what would happen if the horses took the road to his house That would be a trip of thirty-five kilometres in such cold weather!
It was getting late. They rode on and on. At last the horses stopped. Where were they None of them could tell. John looked around. What was that under the tree It was one of their tents!

The horses stopped because ______.

A．it was getting late

B．they were tired after running for a long way

C．they knew that they had got to the camp

D．they had seen John’ s house