When a certain insecticide is sprayed onto a given tree, the active chemical in this insecticide becomes inactive, and non-lethal to insects, after being exposed to the air for 14 days. This chemical becomes active as soon as a tree is sprayed and is no longer considered active as soon as it is ingested by an insect, which dies immediately upon this ingestion. Therefore, the large number of insects that have been observed to die after landing on a tree more than 14 days after this tree has been sprayed must not have died from ingesting the insecticide.

Which of the following, if true, most seriously weakens the conclusion above?

【选项】Within 14 days after a tree is sprayed, rainstorms may carry the insecticide into a nearby stream, preserving the activity of its chemicals until it evaporates into the atmosphere and precipitates down with the rain.