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___you understand this rule, you will have no further difficulty.

     A. Once                       B. At once              C. Only                 D. Only then

___you understand this rule, you will have no further difficulty.

     A. Once                       B. At once              C. Only                 D. Only then

___you understand this rule, you will have no further difficulty.

     A. Once                       B. At once              C. Only                 D. Only then

___you understand this rule, you will have no further difficulty.

     A. Once                       B. At once              C. Only                 D. Only then

___you understand this rule, you will have no further difficulty.

     A. Once                       B. At once              C. Only                 D. Only then