A. When birds exhale and give off oxygen, the air sacs fill the lungs with carbon dioxide at the same time.
B. When birds inhale and give off oxygen, the air sacs fill the lungs with nitrogen at the same time.
C. When birds exhale and give off carbon dioxide, the air sacs fill the lungs with oxygen at the same time.
D. When birds inhale and give off carbon dioxide, the air sacs fill the lungs with more carbon dioxide at the same time.
B. When birds inhale and give off oxygen, the air sacs fill the lungs with nitrogen at the same time.
C. When birds exhale and give off carbon dioxide, the air sacs fill the lungs with oxygen at the same time.
D. When birds inhale and give off carbon dioxide, the air sacs fill the lungs with more carbon dioxide at the same time.
