24. To run a direct current motor, _____ a direct current source.

　　　 A. it must be connected to 　　　　　 B. it must connect to

　　　 C. we must connect it to　　　　　　　 D. we must be connected it to

23. To run machines, _____.

　　　 A. we use electricity　　　　　　　　　　 B. electricity is used　　　

C. electricity must be used　　　　　　 D. we are necessary to use electricity

22. In order to measure matter, ______.

A.　　 units of weight, volume and length are used

B.　　　 units of weight , volume and length should be used

C.　　　 we must have units of weight, volume and length

D.　　 we are necessary to have units of weight, volume and length

21. ______ it rained, we would still go.

　　　 A. Supposing　　　　　　　　 B. Unless　　　　 C. If…not 　　　　　　 D. If not

20. ____ circumstances permit, we will build an aristocratic school in this place.

　　　 A. Provided　　　　　 B. Since　　　　 C. Now that 　　　　 D. Unless

19. ____ he doesn’t come tomorrow, what’s to be done?

　　　 A. Providing 　　　　 B. Since　　　　 C. Now that　　　　　 D. Unless

18. He don’t’ come to see me for ages, ____ he was busy.

A. since　　　　　 B. unless　　　　 C. if…not　　　　　　　　　　 D. until

17.____ this method doesn’t work, let’s try another.

　　　 A. Since　　　　 B. Unless　　　　 C. if… not　　　　　　 D. Until

16.　 ____ he is ill, we’ll do the work for him.

A. Seeing that　　　　 B. No matter how　　　　　 C. However　　　　　 D. Seeing to it that

15.　 Tom, ___ the enemy troops are stationed beyond that mountain before we can make a sudden attack at night.

A. make sure that　　　　　 B. see to it that　　　 C. see to it whether D. make sure whether