Use the Following Data for a Demand Curve $\times$ Quantity Maximized? What Is the Elasticity at That Point

Use the following data for a demand curve.
(A)Use the midpoint formula to calculate the elasticity between a price of $14 and $15.
(B)Use the midpoint formula to calculate the elasticity between $7 and $8.
(C)Because this is a linear demand curve,why does the elasticity change?
(D)At what point is price $\times$ quantity maximized? What is the elasticity at that point?