Correct Option: B. Equilibrium price increases, and equilibrium quantity increases Explanation of the Correct Answer To understand what happens to equilibrium price and quantity when there is an increase in demand while supply remains constant, we need to first define a few key concepts:

1. Equilibrium Price: This is the price at which the quantity of a good demanded by consumers equals the quantity supplied by producers. It is the point where the demand and supply curves intersect.
2. Demand: This refers to how much of a good or service consumers are willing and able to purchase at different prices. An increase in demand means that at every price level, consumers want to buy more of the good.
3. Supply: This refers to how much of a good or service producers are willing and able to sell at different prices. In this scenario, we are assuming that supply remains constant.

Step-by-Step Explanation

1. Initial Situation: Start with a market in equilibrium where the demand and supply curves intersect at a certain price (P1) and quantity (Q1).
2. Increase in Demand: When demand increases, the demand curve shifts to the right. This means that at every price level, consumers are now willing to buy more of the good than before. For example, if the original demand curve was D1, it shifts to D2.
3. Impact on Equilibrium: With the demand curve shifting to the right, the new intersection point with the supply curve (which remains unchanged) will be at a higher price and a higher quantity. This is because:
4. At the original equilibrium price (P1), the quantity demanded has increased (let's say to Q2), but the quantity supplied remains at Q1.
5. This creates a shortage in the market because more consumers want the product than what is available at that price.
6. To eliminate this shortage, sellers will raise the price, leading to a new equilibrium price (P2) where the quantity demanded equals the quantity supplied again, but now at a higher level.
7. New Equilibrium: The new equilibrium will be at a higher price (P2) and a higher quantity (Q2). Thus, both the equilibrium price and quantity increase.

Why Other Options Are Incorrect

• Option A: "Equilibrium price decreases, and equilibrium quantity increases"
• This option is incorrect because an increase in demand leads to a higher equilibrium price, not a decrease. The quantity does increase, but the price does not decrease.
• Option C: "Equilibrium price increases, and equilibrium quantity decreases"
• This option is also incorrect. While it correctly states that the equilibrium price increases, it incorrectly states that the equilibrium quantity decreases. In fact, the quantity increases due to the higher demand.
• Option D: "Equilibrium price decreases, and equilibrium quantity decreases"
• This option is incorrect as it suggests that both price and quantity decrease. An increase in demand cannot lead to a decrease in price; it will always lead to an increase in price and quantity.

Common Pitfalls

• Confusing Demand and Supply Shifts: Students often confuse shifts in the demand curve with shifts in the supply curve. Remember, an increase in demand shifts the demand curve to the right, while an increase in supply shifts the supply curve to the right.
• Ignoring Market Dynamics: Some may overlook the concept of market equilibrium and how shortages or surpluses affect prices. Always consider how changes in demand or supply will create imbalances that lead to price adjustments.

Revision Summary

• An increase in demand shifts the demand curve to the right.
• This results in a higher equilibrium price and a higher equilibrium quantity.
• The supply curve remains unchanged in this scenario.
• Always analyze how changes in demand or supply affect market equilibrium dynamics.

By understanding these concepts, you can better grasp how markets respond to changes in demand and supply, which is crucial for economics exams and real-world applications.