describe double circulation in human beings. why it is necessary?
You might be familiar with the circulation of blood. Now look at this image, this image is a schematic representation of blood circulation.
Here you can observe that the blood passes through the heart twice. Since the blood goes twice through the heart, it is known as double circulation. Warm-blooded animals like mammals have this type of blood circulation.
For the second part of your question you can refer to the answers given by your friends.
@ Sonakshi and gunjan: It's a great answer that you have provided! Keep up the good effort!