A district agriculture office holds that the recorded voice call warning growers of a pest attack is what reduced the loss of the standing crop. Which of the following, if true, most strengthens the office's position?

Four blocks that took the calls in four different months make four small trials, and a fall that begins in each block only once the calls reach it — never earlier — is very hard to credit to weather or to the season, which would have moved all four blocks together. That staggered timing is what turns a coincidence into evidence. A free repeat line describes the service rather than its effect, three earlier years of attack establish only that the pest is a regular one, and the pinned-up note introduces a second possible cause instead of supporting the calls.