For all fires in Chicago last year, data are available on x = number of firefighters at the fire and y = cost of damages due to the fire. The correlation is positive.

(a) Does this mean that having more firefighters at a fire causes the damage to be worse? Explain.

(b) Identify a third variable that could be a common cause of x and y. Construct a hypothetical scatterplot (like Figure 10.1 on page 303), identifying points according to their value on the third variable, to illustrate your argument.