What dose-response curve would be implied in an epidemiological study that shows a standard increase in physical effects for each rise in the dose of a toxic substance? 

A)  a linear dose-response curve 
B)  an exponential dose-response curve 
C)  a threshold dose-response curve 
D)  no dose-response curve

A linear dose-response curve is characterized by a constant increase in effect with the increase in the dose of the substance. This type of curve indicates that there is a direct relationship between the dose of the substance and the response in the population. It does not exhibit a threshold effect, which means that even small amounts of the substance can cause a response. Therefore, a linear dose-response curve would be implied in an epidemiological study that shows a standard increase in physical effects with each rise in the dose of a toxic substance.