Listen! There must be some children ________ songs in our neighborhood.           [ ]

A. singing            

B. to sing          

C. sing                

D. sings

（）是指经济运行过程中繁荣与衰退的周期性交替

A. 经济规律  

B. 经济交替  

C. 经济周期  

D. 经济变动