The Slope of a Demand Curve Is Not Used to Measure

The slope of a demand curve is not used to measure the price elasticity of demand because

A) the slope of a linear demand curve is not constant.
B) the slope of a line cannot have a negative value.
C) the measurement of slope is sensitive to the units chosen for price and quantity.
D) the slope of the demand curve does not tell us how much quantity changes as price changes.