Theory is based on practice and ________ serves practice．

A．in turn          B．by and by           C．by chance         D．in this case

 Theory is based on practice and ________ serves practice．

A．in turn                B．by and by           C．by chance           D．in this case

Theory is based on practice and ________ turn serves ________.

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

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