For ________ goods, market demand is the horizontal summation of individual demand curves and for ________ goods, market demand is the vertical summation of individual demand curves.

A) private; public
B) public; private
C) nonrival; rival
D) nonexcludable; excludable

For private goods, market demand is the horizontal summation of individual demand curves because each unit of the good can only be consumed by one person at a time, leading to a direct addition of quantities demanded at each price level. For public goods, market demand is the vertical summation of individual demand curves because these goods can be consumed by multiple people simultaneously without reducing the amount available to others, thus the willingness to pay at each quantity is added together.