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.

both male and female gametes are produced by meiosis of the germ cells. therefore, both male and female gametes are haploid. one male and one female gamete unite during the fertilization to form the diploid zygote. in seed bearing plants, pollen grain is the male gametophyte, which produces sperm cells.