Answer:
contraction and relaxation of the heart
Explanation:
Answer:
The two membranes pull apart a little and create a small pocket or sack of air. As the developing bird grows, it breathes in oxygen from the air sack and exhales carbon dioxide. Several thousand microscopic pores all over the surface of the egg allow the CO2 to escape and fresh air to get in.
Explanation:
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Fire can not burn without oxygen
They are <span>called </span><span>pathogens.</span>