A district horticulture office holds that the guava fruit fly does not occur in the district, since in twenty years no grower has brought a fly-struck guava to the office. Which of the following is an assumption on which the office's conclusion depends?

The office argues from nothing having been brought in to nothing being there, and that step holds only if a struck fruit would have been brought in. Deny it — growers throw damaged fruit on the heap and never trouble the office — and twenty empty years are consistent with the fly being in every orchard. Published instructions would make such reporting likelier but the argument does not require that any were issued, only that the fruit would have arrived. Whether guava is the district's chief crop bears on how much the question matters, and a report from the adjoining districts, far from being needed, would tell against the office's conclusion.