选用（）、乙烷、（）和乙炔四种特征气体，改变其含量，组成三种比值来判断设备故障的方法

有人说，哲学不是“面包”，不是柴米油盐酱醋茶，不是房子、车子，等等，所以就说哲学没有用处。对哲学和经济关系的正确理解是

A．哲学对社会发展有重要的先导作用

B．哲学是一定社会和时代的经济在思想上的反映，反作用于一定形态的经济

C．哲学只是反映经济，对经济的发展没有影响

D．哲学是对一定社会和时代经济发展的总结，是一种经济理论

       April Fool’s Day（愚人节）, was a day of laughing and jokes. This day is kept in many countries, not only in Britain and the USA. This is a day to play jokes and make people laugh. Nobody knows when was the beginning of this custom. Some people connect it with the end of winter and the return of spring which make people merry and ready to play jokes.

In Scotland young people were sent for hen’s teeth or bird’s milk and everybody laughed when they could find such things. In the USA and Britain some people could place a sign on a person’s back with the words “push me!” Children often tell a grown-up that his sock is torn or he has something black on his face, and then shout “April Fool!” There is also the old purse（钱包） trick. A purse is left lying in the street, but when someone wants to take it, it is quickly pulled back by a string which the hidden joker holds in his hand. Or the purse may be filled with stones. Sometimes invitations are sent to people, asking them to come and visit somebody, but when they come they see that nobody expects them. Some people like to telephone to the zoo on the day and ask for Mr. Fish, Miss Fox or Mrs. Cat. All these jokes are very old but still they make people laugh.

小题1: ____________knows when the custom of April Fool’s Day started．

A．One

B．Anyone

C．Nobody

D．Someone小题2:Many tricks are played on April Fool’s day except ____ ．

A．phoning the zoo

B．placing a sign on someone’s back

C．playing cards

D．leaving a purse in the street小题3:The purpose of playing jokes is to _____ ．

A．for fun

B．make people laugh

C．make fool of oneself

D．fool someone小题4:The best title of the passage is ____ ．

A．How people in the west spend their April Fool’s Day

B．Customs are different

C．How British people celebrate their holidays

D．How April Fool’s Day comes about

为什么我国规定110kV以上的高压电网和380/220V的低压电网要采用大电流接地系统？各有什么优点？

属于颞下颌关节紊乱病不可逆性保守治疗的是

A．药物治疗

B．物理治疗

C．封闭治疗

D．咬合导板治疗

E．正畸治疗

下面哪个元件在色氨酸操纵子的调控中发挥作用（）。

A.增强子

B.转录子

C.弱化子/衰减子

D.顺反子