Arrange the sentences in the correct order to form a coherent paragraph.
A. Young birds frequently make their first flights with their parents out of sight.
B. Even young birds brought up in artificial environments will build the proper kind of nest for their species when the time comes.
C. More extraordinary than the fact that a bird is able to fly untaught, is that it is able to build a nest untaught.
D. The most obvious way in which birds differ from humans in behavior is that they can do all that they have to do, without ever being taught.