Final answer:
The law of demand states that a higher price leads to a lower quantity demanded and a lower price leads to a higher quantity demanded.
Step-by-step explanation:
The law of demand is a fundamental principle in economics that describes the inverse relationship between the price of a good or service and the quantity demanded by consumers. According to this law, other things being equal, when the price of a good increases, the quantity demanded decreases, and conversely, when the price decreases, the quantity demanded increases. Therefore, the correct answer to the student's question is c. price causes quantity demanded to decrease.