With no explanation, label text_A→text_B with either "DON'T KNOW", "NO" or "YES".
text_A: Early in the Cenozoic, following the K-Pg event, the planet was dominated by relatively small fauna, including small mammals, birds, reptiles, and amphibians. From a geological perspective, it did not take long for mammals and birds to greatly diversify in the size of the dinosaurs that had dominated during the Mesozoic. Some flightless birds grew larger than humans. These species are sometimes referred to as "terror birds," and were formidable predators. Mammals came to occupy almost no niche (both marine and terrestrial), but some grew very large, attaining sizes not seen in most of today's terrestrial mammals.
text_B: Does the K-Pg event allow mammals to benefit, in that dinosaurs can't compete with them?
YES.