(a) Conceptually what is the difference between a variable that measures a flow and one that measures a stock?  (b) What is meant by the growth equation in resource economics? (c) What is the formula...

(a) Conceptually what is the difference between a variable that measures a flow and one that measures a stock?

(b) What is meant by the growth equation in resource economics?  (c) What is the formula for the growth equation in the case of a non-renewable resource?  (d) What is the intertemporal externality that arises in the case of a non-renewable resource?