Great question. You're right; (C) doesn't say anything about how many readers returned the survey. However, this doesn't matter because (C) tells us something even more important: the survey accurately reflects the feelings of the readership as a whole. The real reason we'd care about how many survey readers returned the survey would be to see how closely the survey's population represents the readership as a whole. (C) doesn't tell us how many readers returned the survey, but it assures us that the survey's results mirror the readership's preferences, and this is what really matters to us.
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