The price elasticity of demand equals 1:
A.whenever the slope of a straight-line demand curve equals zero.
B.whenever the slope of a straight-line demand curve is less than 1 in absolute value.
C.at the midpoint of a straight-line demand curve.
D.whenever the slope of a straight-line demand curve is greater than 1 in absolute value.
正确答案：at the midpoint of a straight-line demand curve.
