<span>Pleural sac is a two
layered membrane formed when a serous membrane known as pleura folds back onto
itself. Thus, the pleura invest the lungs and line the walls of the thoracic
cavity. However, the potential volume of the thoracic cavity increases during
inhalation.</span>
Answer:
The answeer is both B and D.
B, (layer C is the youngest layer) :)
The answer to your question is:
<span>Stomata allow gasses to flow in and out of a leaf.
</span>
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Answer:
I believe this is a pacemaker. It controls the heart when it starts to fail.
Explanation: