题目：
Theory is based on practice and ________ serves practice.
A．in turn
B．by and by
C．by chance
D．in this case
答案：
答案：A
短语辨析. A.依次;轮流 B. 不久以后C. 意外的 D.既然这样.句意:理论基于实践并反过来为实践服务.结合语境可知选A.
Theory is based on practice and ________ serves practice.
A．in turn
B．by and by
C．by chance
D．in this case
答案：A
短语辨析. A.依次;轮流 B. 不久以后C. 意外的 D.既然这样.句意:理论基于实践并反过来为实践服务.结合语境可知选A.
2CO2+3H2O，其中x、y的数值分别是[ ]