Answer:
The right answer to this question is: Have mammary glands and have a four chambered heart.
Explanation:
The mammary glands are very important for all the mammals because, without the mammary glands, there's no way to feed the next generation of the species, commonly it's very hard for a baby cat, for example, to search for food by its own, and even though if this happens, will the resources be enough to fulfill all its needs? Probaly not.
The four chambered heart is very important too because we have two kinds of blood, the "dirty" one, and the "clean" one, in other words, venous blood and arterial blood. Without these four chambers, the blood would pretty much be mixed inside the heart, and that's not an advantage for mammals because by doing this, the process is not as efficient as it needs to be.
