Question

Which pair of variables has a clear causal link between the two named variables?(Please Help)
A: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
B:the number of notebooks sold and the number of pens sold in a stationery store
C:the number of air conditioners sold per day in a city and the daily temperature
D: the number of hours a person can swim and the depth of the pool