Read the following passage and then identify the main idea:

Dolphins are mammals that live in the ocean. Mammals are different from fish, reptiles, or birds. As mammals, dolphins breathe oxygen, even though they live in water. Because it is a mammal, a dolphin mother gives birth to a live baby, unlike reptiles and birds, which lay eggs. A dolphin mother also feeds her baby milk like other mammals.

What is the main idea of this passage?

Dolphins are mammals.

Mammals are different than fish, reptiles or birds.

Dolphins breathe oxygen, even though they live in water.

A dolphin mother also feeds her baby milk like other mammals.