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

Which letter in the diagram indicates the type of soil that supports grasses and small shrubs?NAR: SOIL

CFCs remain in the troposphere because they are ____________________ in water and chemically unreactive.

_________________________ can make the detection of a cancerous tumour difficult because they can travel to other parts of the body.

At which age are males more likely to die from AIDS than females?