A municipal health officer reports that the fogging drive against mosquitoes began on 1 August and that fever cases reported in the town fell steeply over August and September; he holds that the fogging brought the fever down. Which of the following, if true, most weakens the officer's conclusion?

A cause cannot be credited with a fall that was already under way before it began and that did not quicken when it began. The August and September figures are simply the June and July figures continued, and whatever was bringing the fever down in June is bringing it down still. Who was engaged to do the fogging and which institutions report the cases were the same before and after; and that the nuisance peaks in July and August describes the mosquitoes without explaining why the fall neither began nor steepened on 1 August.