Question

HELP plz!!!

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