A mother notices that when she divides brownies equally between her two children and gives each child her share on a separate plate, the brownies last a long time. But when she gives her children a plate to share, the brownies are gone pretty quickly. The mother concludes from this that brownies given on a single plate are
a. excludable and nonrival
b. nonexcludable and rival
c. excludable and rival
d. excludable but they might either be rival or nonrival
e. nonexcludable and nonrival