Many egg-laying animals merely lay their eggs and leave. With turtles, for instance, and horseshoe crabs, the eggs hatch and the little ones are on their own. The current theory about birds is the earliest birds did just that when they were cold—blooded creatures living in warln places. However, when they became warm-blooded creatures living in cold places, they had to remain on the eggs to keep them warm. For this, they needed a place—a nest. Very likely, the first nest was just primitive hole scraped in the ground. Even now many species still lay eggs in this sort of crude nest. Primitive nests found on the ground were fine for some birds, but others began to elevate their nests in branches, perhaps to avoid predators. These early elevated nests were probably loose platforms of sticks and dried leaves. The type is still built by some birds today. The latest development in nests, the most recent version, so to speak, is the cup-shaped nest. This is the one we regard today as the typical bird’s nest, you know, like a robin’s nest. According to the current theory, the earliest birds laid their eggs and left if
A. they were warm-blooded animals living in cold places. B. they were warm-blooded animals living in warm places. C. they were cold-blooded animals living in cold places. D. they were cold-blooded animals living in warm places.