Suppose that x is the price of one brand of gasoline and y is the price of a competing brand. Then q₁, the quantity of the first brand sold in a fixed time period, depends on both x and y, so q₁ = f(x, y). Similarly, if q₂ is the quantity of the second brand sold during the same period, q₂ = g(x, y). What do you expect the signs of the following quantities to be? Explain.
(a) ∂q₁∕∂x and ∂q₂∕∂y 

(b) ∂q₁∕∂y and ∂q₂∕∂x