The answer choice which reduces friction around the lungs as they expand and contract is; Choice C; Pleural fluid.
What reduces friction around the lungs as they expand and contract?
A membrane with two layers, termed the pleura surrounds the lungs. A slippery liquid called pleural fluid acts as a lubricant to reduce friction in the space between those two layers, called pleural cavity during breathing.
Remarks:
A. Surfactant
B. Synovial fluid
C. Pleural fluid
D. Mucus
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