Which best explains the law of demand?
 A. 
 As the amount of a good increases, the price that consumers are willing to pay decreases.
 B. 
 As the price of a good decreases, the amount that consumers are willing to purchase decreases.
 C. 
 As the price of a good decreases, the amount that consumers are willing to purchase increases.
 D. 
 As the amount of a good increases, the price that consumers are willing to pay increases.