After a pedestrian crossing with a raised table was built outside a school gate, the number of pedestrians struck on that stretch of road fell by half. The town engineer argues that the crossing is what brought the number down. Which of the following, if true, most strengthens the engineer's argument?

Fewer vehicles, better lighting and a change in the hours at which children cross are the three obvious ways the count could have halved without any help from the crossing. Showing that none of them moved removes the rivals and leaves the crossing as the only candidate. The material of the table, where the pupils come from and how long the work took bear on the crossing itself, not on what else might explain the fall.