The correct answer is D. Monotremes are unique mammals because they lay eggs.
Monotremes are a group of mammals that lay eggs instead of giving birth to live young. They are the only mammals that do this. Monotremes are found in Australia and New Guinea. There are only five species of monotremes: the platypus, the short-nosed echidna, the long-nosed echidna, the western long-beaked echidna, and the eastern long-beaked echidna.
Monotremes are unique in many ways. They are the only mammals that have a cloaca, which is an opening that is used for both urination, defecation, and reproduction. They are also the only mammals that have electroreceptors, which are organs that allow them to sense electrical fields.
Monotremes are an important part of the Australian ecosystem. They help to control populations of insects and other small animals. They are also a popular tourist attraction.
Here is a brief explanation of each option:
- Option A: Monotremes do have hair, like other mammals. However, their hair is very fine and sparse.
- Option B: Monotremes do not give birth to live young. They lay eggs, like reptiles and birds.
- Option C: Monotremes do secret milk to feed their young. However, they do not have nipples. Instead, they secrete milk through pores in their skin.
- Option D: This is the correct answer. Monotremes are unique mammals because they lay eggs.