about demand & supply!!! (10 points)

I think demand curve here means individual demand curve while market demand means the horizontal summation of individual demand curves at each price, thus forming the market demand curve. A change in demand means the movement of the demand curve (leftwards shits or rightwards shift of the demand curve): if economic bloom, then demand curve would shift to the right but the fall in price of substitute goods would reduce the demand, hence, shifting the demand curve to the left. On the contrary, change in quantity demanded is the movement along the demand curve, solely caused by change in price.