Please help...? I've been helping so many people and I could really use some help myself...

Today, it’s widely accepted that all of the two-legged meat-eating dinosaurs known as theropods— including T. rex and Albertosaurus—had at least very simple fuzzy feathers covering their bodies. According to the tree you built, what distinguishes the feathers of modern birds and Archaeopteryx from the feathers of other theropods?
a. Barbs
b. Fibers
c. Filaments
d. Shafts