The order cetacea includes modern-day whales, dolphins, and porpoises. these marine mammals share common characteristics that have led scientists to make a conclusion about their evolutionary history. cetaceans:

lack gills and must breathe air
give birth to live young rather than lay eggs
have fin bones that are similarly shaped to limb bones
have spines that flex up and down rather than side to side
the characteristics of cetaceans provide evidence that

a. they evolved from land mammals.

b. the theory of evolution is incorrect.

c. they evolved from air-breathing fish.

d. the bodies of their ancestors never changed.

The correct option is A. they evolved from land mammals.

Explanation:

The process of evolution explains how closely one organism is to another based on their common ancestors. Scientists believe that those organisms which show many similarities in their structures or functions might have evolved from a common ancestor in the past.

As in the above-mentioned question, scientists have observed that cetaceans lack gills and breath air, this is a property of land mammals. The cetaceans have fin bones which are similar to the limb bones. As limb bones are present in land mammals, hence they cetaceans and land mammals might have common ancestors in the past.

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if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.