22. Edison’s theory________ to be correct.

　 A. turned out 　　　　B. was turned out　　 　C. turned on　 　　　D. turned off

21. ---How did you pay the workers?

　 ---As a rule, they were paid ________.

　 A. by hour　 　　　　B. by an hour　 　　　C. by the hour　 　　D. by hours