Which of these are common to all mammals?

type of reproductive system, brain size, skin

digestive system, heart, lungs

digestive system, number of toes, blood

kidneys, liver, skeletal structure

digestive system, heart, lungs

Mammals are warm blooded vertebrate animals, that possess hair or fur, they give birth to their young ones, and females secrete milk for the nourishment of the young ones. The characteristics which are common in all mammals are:

Digestive system: All mammals exhibit similar type of digestive system which includes alimentary canal and digestive glands which secret digestive juice to digest food.

Circulatory system: All mammals exhibit double circulatory system, which means all mammals have two chamber heart, so that the blood can be pump out to the lungs and the entire body.

Lung system: All mammals exhibit two lungs that are located on either sides of the heart.

the cost of mining is extremely low compared to the money earned from mining, creating income to improve the environment.