One of the Finest Available Sequences of Fossils Shows How

One of the finest available sequences of fossils shows how horses have changed slowly and by subtle steps from small shrub- browsing ancestors to the large,grass- grazing modern horse.A large number of fossil species have been named,and it is often difficult to decide on the identity of a fossil horse because transitional forms are common.This record of evolution best fits the idea of

A) punctuated equilibrium.
B) the gradual model of speciation.
C) hybrid breakdown.
D) adaptive radiation.