Recent years have seen the introduction of Uber and Lyft$,$ which enable customers to find a ride using an app on their smartphone. In many cities, the two services undercut traditional taxi fares, leading to phenomenal growth. Uber and Lyft are best described as the

a$.$ environmental marketing model.

b$.$ sharing economy economic model.

c$.$ customer lifetime value model.

d$.$ marketing concept.

e$.$ sustainable marketing model.