A、On the ground. B、In cold places. C、On the highest branches of trees. D、Inside tree trunks. A题目问的是鸟类最开始是在哪里筑巢的。由定位句可知，最开始的鸟巢是在地

Many egg-laying animals merely lay their eggs and leave. Turtles for instance, the eggs hatch on their own. The current theory about birds is that the earliest birds did just that when they were coldblooded creatures living in warm places. However when they became warm-blooded creatures living in cold places they had to remain on the eggs to keep them warm. The process we call incubation. For this they needed a place to nest. Very likely the first nests were just primitive depressions scrape into the ground. Even now many species still lay eggs in this sort of crude nests. In fact every spring a mother killdeer lays her eggs in some pebbles along the edge of the parking lot just outside this building. Primitive nests on the ground were fine for some birds but others began to elevate their nests in branches perhaps to avoid predators.
    Q19: What evolutionary change in birds led to nest building?
    Q20: According to the speaker, where were the first bird’ s nests located?
    Q21: What is the killdeer’ s nest in parking lot an example of?