A-Weight of water collapses lime stone below it, water drains into the sinkhole
So many new species were being discovered, it was impossible to arrange them all in a hierarchy based on relative complexity of species.
Answer:D
Explanation: The pulmonary air sacs of birds only permit a unidirectional flow of air through the lungs. Unidirectional flow means that air is inhale and exhale through one trachea alternatively. This means the bird lungs only receive largely fresh air which has a high oxygen content during inhalation and do not receive fresh during exhalation. While air flow bidirectional in mammals.
