Economists believe the law of demand, that people are willing and able to buy more at lower prices than at higher prices, because:

it seems to describe how people act.
the definition of demand specifies that it be this way.
it is an equilibrium position.
producers will try to maximize profits.

"I do not care how much gasoline costs. I have to buy five gallons a week." If the speaker of this statement acts as he says he does, his demand curve for gasoline:

is positively sloped.
is negatively sloped.
is vertical.
does not exist.

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