Question

MEDALS! HELP! In which of the scenarios can you reverse the dependent and independent variables while keeping the interpretation of the slope meaningful?
As the number of cigarettes smoked in a day increases, the risk for lung cancer increases.
As the age of an infant increases, the height of the infant increases.
As the number of vehicles in a city increases, the amount of pollution in the city increases.
As the concentration of salt in water increases, the density of water increases.
As the number of calls made by a cell phone increases, the cell phone bill amount increases.