You are presented with the quiz "Why do dogs play?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "playing is just one of many ways that mammals and other animals learn how to behave" and "cats are Mammals that Shed Dogs are Mammals that Wag their tails". So, what's the best answer to the question?

pick from the following.
+ To keep their body heat.;
+ To win a fight;
+ it keeps an organism warm;
+ increases a body's strength;
+ To learn to behave;
+ allow growth;
+ To become dominant;
+ To spend time alone;
The answer is:
To learn to behave