Active and passive voices are the form of verbs. The correct blanks are 'had been complained about' and 'had been found.'
Active voices are the grammatical voice that represents the object performing actions of the sentences and the passive voice receives the action of the sentence.
The first blank will be 'had been complained' about as, it will use past passive voice, and the other blank 'had been found' is also the past tense as the information is already known.
Therefore, the correct blanks are 'had been complained about' and 'had been found.'
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