Question

Which pair of variables has a clear causal link between the two named variables?
the number of hats sold and the number of mittens sold in a city
the amount of butter and the amount of bread consumed by a family
the number of notebooks sold and the number of pens sold in a stationery store
the number of air conditioners sold per day in a city and the daily temperature
the number of hours a person can swim and the depth of the pool