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4 is the correct answer 

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The branches contain blood capillaries beneath a thin epithelium that separates the blood from the water, allowing oxygen and carbon dioxide to pass through easily. 4 seems correct

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I believe the answer should be 4.

A and B are related. The gill filaments spread out to increase the surface area for gaseous exchange. Increased surface area → increased rate of respiration.

D is also correct. At PSLE level, student only needs to liken the gills (gill filaments) to the lungs (air sacs) of mammals and draw similar comparisons. Other complex mechanisms like counter-current exchange and the gill arches are not required.

Mammals suck in air containing oxygen through the nasal and oral cavities . Fish suck in water containing dissolved oxygen.
The lungs take in oxygen and facilitate the absorption of oxygen into the bloodstream. The gills process the dissolved oxygen and facilitate the absorption of oxygen into the bloodstream.
The circulatory system carries oxygen to all cells in the body and carries carbon dioxide back to the heart and eventually back to the lungs. The circulatory system carries oxygen to all cells in the body and carries carbon dioxide back to the heart and eventually back to the gills.
The lungs expel the carbon dioxide from the bloodstream out of the lungs and into the air through the nose. The gills expel the dissolved carbon dioxide from the bloodstream out of the gills and into the water through the gill flaps.

 

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Thanks for your reply. 

The question is why there are many blood vessels in the gills. It is for A & B to happen. 

Having many blood vessels does not impact D. 

In that perspective, 1 will be the more appropriate answer. is it not ?

 

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Sorry, did not read the part about blood vessels for some reason.

That said, the blood vessels do allow the carbon dioxide to be expelled from the bloodstream to the water. They definitely have the function to allow dissolved carbon dioxide to be released out into the water through the epithelial tissues.

All gases involved in respiration, whether oxygen or carbon dioxide, must pass through the bloodstream. This is similar for both mammals and fish.

So my answer would still be 4.

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ok, thanks.

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