A new variety of Lays is initially introduced in Phoenix, Arizona; Cleveland, Ohio; Battle Creek, Michigan; and Peoria, Illinois. After its initial success, Frito$-$Lay markets the new Lays in those entire states, then the adjacent states, and finally the entire country. Frito$-$Lay is using a $\_\_\_\_$ for its new variety of Lays.

a$.$ test market expansion

b$.$ rollout approach

c$.$ trickle$-$down approach

d$.$ multistage introduction

e$.$ mass commercialization