I am not understanding this diagram about the elasticity and the marginal revenue, please explain this step by step , thanks !

E? t 3 51\".. CL. 0 (units/wk) FIGURE 13.6 Marginal Revenue and Position on the Demand Curve When Q is to the left of the midpoint (M) of a straight-line demand curve (for example, Q = Q0), the gain 'om added sales (area B) outweighs the loss 'om a lower price for existing sales (area A). When Q is to the right of the midpoint (for example, Q = Q1), the gain 'om added sales (area D) is smaller than the loss 'om a lower price for existing sales (area C). At the midpoint of the demand curve, the gain and the loss are equal, which means marginal revenue is zero