As is known to us all, theory is based on practice and __________ serves practice.

A. by chance　　    　　   B. in turn 　　　　　　　　　　 C. in return　　　　 　　   D. by turn

A. by chance　　    　　   B. in turn 　　　　　　　　　　 C. in return　　　　 　　   D. by turn