What is a skeleton and give two characteristics of a mammal​

A skeleton is a structural frame that supports an animal body.

The skeletal system of mammals and other vertebrates is broadly divisible functionally into axial and appendicular portions. 

There are two major types of skeletons: solid and fluid. Solid skeletons can be internal, called an endoskeleton, or external, called an exoskeleton, and may be further classified as pliant (elastic/movable) or rigid (hard/non-movable).

Fluid skeletons are always internal.

The presence of hair or fur.

A four-chambered heart.

well I know the skeleton is the bone structure in some living organisms if there a vertebrate

answer; polycythemia is the correct answer;

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.