Why is it that the industry demand curve slopes downward when the demand curves faced by individual firms in perfectly competitve markets are horizontal?

The industry demands the curve that slopes downward because if the price of the goods increases then the quantity will decrease and this is demonstrated in the Law of supply and Demand

Explanation:

The changes in the curve indicate the increase and in the decrease of the products and their demand. When a market is perfectly competitive the demand curve will be downward sloping

This indicates the fact that if the price of the ordinary good increases the quantity or the demand for that good decreases and finally the firm will not sell any products It will exhibit a horizontal line because the in the individual demand curve the exact substitutes are found in the market