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 birds, reptiles, and amphibians. From a geological perspective, it did not take long for birds to greatly diversify in the absence of the mammals that had dominated during the Mesozoic. Some flightless birds grew larger than dinosaurs. These species are sometimes referred to as "terror birds," and were formidable predators. birds came to occupy almost every available niche (both marine and terrestrial), and some also grew very large, attaining sizes not seen in most of today's terrestrial birds.
text_B: Does the K-Pg event allow mammals to benefit, in that dinosaurs can't compete with them?
YES.