Why does an economist create a market demand curve? to predict how people will change their buying habits when prices change to have an idea of how a market would change if conditions in an area changed to show how various conditions can change the demand for a good to learn what demands the market will make under unusual conditions?

I believe economists create demand curve to predict how people will change their buying habits when prices change. A demand curve is plot of Quantity of products or services demanded in the market against the prices for a given period of time. The price appears on the horizontal axis and the quantity demanded on the y axis. The demand is the quantity of goods or services that the consumers are willing and able to buy at a given period of time. 

It is answer choice (a.) true.