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I decided to concentrate all my efforts _____ the complicated problem.

A. to solve
B. to be solved
C. on solving
D. in solving
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科目：高中英語 來源：0109 同步題 題型：單選題

I decided to concentrate all my efforts _____ the complicated problem.
[     ]
A. to solve
B. to be solved
C. on solving
D. in solving

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