Bicarbonate ions which migrate from red blood cells into the surrounding fluid are replaced by ions.

chloride ions

Explanation:

This movements of  chloride ions  into the blood plasma to replace the outward diffused bicarbonate ions is  called  chloride shift, It  occurs  to restore the blood ionic balance altered by the bicarbonate ions diffusion out into plasma

The same amounts of Chloride ions replaced the lost amount of  bicarbonate ions

estuaries are the partially enclosed coastal bodies of brackish water formed at the mouth of sea where river meets an ocean. it is a transition zone between river and sea water so estuaries are influenced to both marine environment such as tides, waves and to riverine such as flow of fresh water and sediments. the mixing of sea and fresh water provide high level of nutrients, making estuaries a high product habitats.

many estuaries suffers degradation from a variety of human activities like erosion from deforestation, overgrazing, overfishing, drainage and eutrophication.

so from the above statement answers will be

1. false as estuaries are formed due to both brackish and freshwater.

2.true, due to high productivity estuaries are breeding ground for many marine species.

3. direct and indirect activities of man that contribute to pollution and deposition of sediments.

d. planting trees

the trees take in carbon to make oxygen.