I think the answer is C Spiral, I haven't learned this since 5th grade though so I might be wrong.
People have grown completely immune to smallpox, meaning that the vaccine has done it's job and is no longer needed.
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.
Answer:
Many people and even scientist believed in spontaneous generation because it explained such occurrences as the appearance of maggots on decaying meat. By the 18th century it had become obvious that higher organisms could not be produced by nonliving material.
<span>The process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. </span>
