​One reason demand curves slope downwards is

A) ​Marginal value increases with each purchase
B) Marginal value declines with each purchase
C) Total value declines with each purchase
D) ​All of the above

Marginal Value

The extra benefit or pleasure derived from using or making an additional unit of a product or service.

Demand Curves

Graphs that illustrate the relationship between the price of a good and the quantity demanded by consumers at those prices.