Decide which of the choices given below would correctly complete the passage if inserted in the corresponding blanks. Mark the correct choice for each blank.

When we ___1___the subject of alternating current, we said that the great use made of A.C. supply systems is due to the ease with___2___  an A. C supply can be stepped up or ___3___in voltage. ___4___ this we use a static transformer. This device transforms low-voltage energy into high-voltage energy and___5___.

Transformers are also required for the___6___ of most luminous discharge tubes and for bell systems in home. They are found in radio and television___7___, with several different ___8___often used in the same circuit. Telephone systems use large numbers of transformers, as ___9___X-ray equipment; so we may say that ___10___electricity is used, transformers are used too.
1.a.introduce  b. introduced   c. introducing   d. to introduce
2.a. what      b.which        c. whom          d. why
3. a.down      b.up           c. below         d. above
4.a.To do       b. Do         c. Did            d. Doing
5.a.vice versa   b. versa vice    c. on the contrary   d.the same as
6.a. operate     b. operating    c.operation        d. operator
7.a. equipment  b. department   c. apartment       d. device
8.a. types      b. kinds        c. ways           d. methods  
9.a. is         b.did          c. does            d. doing
10.a. when     b. what        c.which           d.where
Key: 1. b  2. b  3. a  4. a  5. a  6. c  7. a  8.a  9.c  10.d