Along any downward-sloping straight-line demand curve:

a) both the price elasticity and slope vary.

b) the price elasticity varies but the slope is constant.

c) the slope varies, but the price elasticity is constant.

d) both the price elasticity and slope are constant.

e) at the origin, the price elasticity is equal to one.

b) the price elasticity varies but the slope is constant.

Reason: In case of a linear demand curve, the price elasticity changes along the demand curve while the slope of the curve remains constant throughout its length. The price elasticity of demand in a linear demand curve can be measured by calculating the lower segment divided by the upper segment. The upper potion of the demand curve is thus elastic in nature, the lower potion of the demand curve is inelastic in nature and the middle portion is unitary elastic in nature.