Select ALL correct answers. All mammals share these characteristics:
A) hair and mammary glands
B) placenta and hair
C) endoskeleton and amniotic egg
D) endoskeleton and opposable thumbs
E) jaws and dorsal nerve cord
Both answers A and E are correct. All mammals have hair and mammary glands. Hair is for protection, warmth and camofluage. All living mammals have a jaw joint. The lower jaw jones has teeth and the squamosal bone meets that and form the jaw joint. Mammals all have sweat glands and those can be modified to produce milk to nourish the young mammals. They are known as mammary glands and are functional in female mammals after they give birth to nurse their young with. While all mammals have an endoskeleton, they don't all have a placenta. Marsupials like kangaroos contain a pouch and duckbilled platypus are egg-layers. Primates are the mammals with opposable thumbs but these are not found in other groups.
The only correct answers are A and E. All mammals do have hair, and mammary glands. Mammals also have jaws, though spiny anteaters have toothless jaws. This renders them incapable of chewing their prey. Instead, they swallow ants and termites whole.
B is incorrect because not all mammals have a placenta. Marsupials, like Kangaroos, skip the placental stages of pregnancy and instead the young develop inside the mother's pouch. And some mammals don't even do that, the duck-billed platypus is oviparous i.e. it lays eggs. That's right platypus lays eggs like birds and reptiles, but it is still a mammal because after the eggs hatch, the mother feeds the babies with milk from her mammary glands.
C. is also incorrect because as noted, some mammals are oviparous, and lay eggs, amniotic egg is characteristic of viviparous animals (i.e. those giving birth to live young).
D. Is incorrect because not all mammals have opposable thumbs. Humans, primates (apes and monkeys), and Koalas all have opposable thumbs; but cows, deer, and whales are all mammals and do not have opposable thumbs.
A, C, and E are correct.